Rsync with space in the directory name

I was looking for a way to include a directory name with space using rsync. So I google it, found a brunch of suggestions, which turned out that NONE of them really works. As always, I ended up coming the solution on my own. Here is how I did it:

rsync -avr '[email protected]:/directory\ abc/' /directory\ abc/

That’s it. Enjoy!

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Running ZFS on Linux: Things you should know and be aware of

ZFS is the next generation file system. Unfortunately, it won’t be shipped with Linux because of legal/licensing issues. Fortunately, it is possible to install it (ZFS on Linux) in few commands. Since 2013, I have set up a number of Linux (CentOS/RHEL) servers with ZFS for use in a high traffic production environment. They include high-end commercial grade server (Xeon-based + ECC memory), gaming quality desktop (i7-based) and entry-level consumer grade computer (i3). In this article, I will discuss about what I have learned from my experience.

Warning on ZFS on Linux

ZFS on Linux is not a robust solution to implement ZFS on Linux because it has a very important (and impossible) requirement: The system will never get updated and rebooted. If you cannot meet this requirement (obviously), be prepare to spend tons of hours to fix the problem and get your data back. See how I fix the problem created by ZFS on Linux here. If you prefer rock solid and reliable way, you have to go with *BSD or Solaris.


Life is short. If you don’t want to waste your time to go through the entire article, here is my advice: Use FreeBSD (or *BSD) if possible. Using ZFS on Linux is like putting a giraffe in the wild Alaska. It is not going to work. However, we may want to stick with one operating system for server for various reasons. Therefore, I’ve come up some advice for you if you really want to run ZFS on Linux:

  • Use a commercial grade server when it is possible. A bare-bone entry-level Dell Power Edge T110 II (starting from US$300) is sufficient to run ZFS as a low traffic, light load, nightly backup server. Consumer grade computer is not recommended for use in ZFS/Linux. If you really need one, get a computer with gaming quality grade components and always back up the data on a different server.
  • Linux kernel plays an important role to ZFS. Try to use v.3 (e.g., RHEL 7) when possible. Using ZFS with v. 2.6 (e.g., RHEL 6) may cause some unexpected problem to non-commerical grade hardware. As of October 2019, I cannot make version 4 (e.g., install via kernel-ml or CentOS 8) works with ZFS on RHEL 7:
    Loading new spl- DKMS files...
    Building for 4.11.2-1.el7.elrepo.x86_64
    Building initial module for 4.11.2-1.el7.elrepo.x86_64
    configure: error: unknown
    Error! Bad return status for module build on kernel: 4.11.2-1.el7.elrepo.x86_64 (x86_64)
    Consult /var/lib/dkms/spl/ for more information.
  • Set up your ZFS with the hard drive identifier (e.g., /dev/disk/by-id/someid), not the generic device id (e.g., /dev/sda).
  • You may lose some storage space (smaller than 1%) comparing to the same setup in FreeBSD. But the amount is trivial.
  • If you already install ZFS on Linux, try to exclude the kernel from system update. The system will not load the ZFS after reboot, and it will take some extra work to get ZFS running again.
  • Some Linux distributions such as CentOS 7 will not load ZFS at the boot time. You can solve this problem by using cron job. If you have other services (e.g., MySQL, NFS, Apache) that depends on the ZFS, you will need to restart them.
  • Bookmark this ZFS emergency recovery guide. Trust me, you never know when your ZFS on Linux decide to stop working.

Do not update the kernel automatically

I’ve wrote an article on how to rescue your ZFS file system after updating the kernel. Please click here for details.

ZFS is not native in Linux. The whole idea of ZFS on Linux is nothing more than a brunch of modules being injected to the kernel, such that the kernel will load the ZFS at boot time. This is a fantastic idea because it will not introduce the performance problem like ZFS/FUSE (running on the user land, i.e., very slow). However, there is a potential problem here. This “injection” only happens when a ZFS module (zfs-kmod) is needed to be installed or updated. During this process, the system will download the latest copy of the zfs-kmod and injecting it to the current running kernel. See the problem here?

That being said, running root (/) on ZFS in Linux is a very very bad idea. You will not be able to access anything when the ZFS is not available at the kernel level.

So we have four different situations here after hitting the update command:

Kernel has new update
Kernel has no update

zfs-kmod has new update
Yes. Your ZFS will be available after the reboot.
Yes. Your ZFS will be available after the reboot.

zfs-kmod has no update
No. Your ZFS will not be available after the reboot.
Yes. Your ZFS will be available after the reboot

In general, if you really need to update the kernel, you will need to update the kernel first, reboot to the new kernel (ZFS will be missing), and re-run the process such that ZFS module will be injected to the new kernel. Some people may recommend to uninstall the zfs-kmod and reinstall it again. Unless you have a very strong reason to use the latest kernel (e.g., you’ve got plenty of spare time), otherwise I won’t recommend doing it because the whole process is a pain.

Another thing you can do is to disable the auto update. Only update the system when there is a new update for both kernel and the zfs-kmod. Then you can update the kernel first, reboot, and then update the zfs-kmod after the reboot. However, keep in mind that you will run into some problem eventually. Many packages depend on the newer kernel, if you try to update the system, it will complaint because you will need to update the kernel first before updating those packages. You can get around by skipping the broken packages (yum update –skip-broekn).

In my settings, I simply exclude the kernel from the update. That way I only need to work with one kernel, and I know that that particular kernel knows how to handle ZFS module.

sudo nano /etc/yum.conf 


In case you are running into trouble, i.e., ZFS is missing in the latest kernel, you can try doing the following:

Before running the following commands, make sure that you know what you are doing.

#Make sure that you reboot to the kernel you want to fix.
#Find out what is the current kernel
uname -a
Linux 3.10.0-514.2.2.el7.x86_64 #1 SMP Tue Dec 6 23:06:41 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

#In my example, it is:

#Basically we want to remove the following files:
ls -al /lib/modules/your_new_kernel/extra
-rw-r--r-- 1 root root 344K Dec 12 15:58 splat.ko
-rw-r--r-- 1 root root 167K Dec 12 15:58 spl.ko
-rw-r--r-- 1 root root  14K Dec 12 16:02 zavl.ko
-rw-r--r-- 1 root root  75K Dec 12 16:02 zcommon.ko
-rw-r--r-- 1 root root 2.2M Dec 12 16:02 zfs.ko
-rw-r--r-- 1 root root 130K Dec 12 16:02 znvpair.ko
-rw-r--r-- 1 root root  34K Dec 12 16:02 zpios.ko
-rw-r--r-- 1 root root 324K Dec 12 16:02 zunicode.ko

#If you have no extra modules installed other than ZFS and SPL, you can run the following:
sudo rm -Rf /lib/modules/*/extra/* 

#Otherwise just remove the files one by one.

#And we want to do the same thing to the weak-updates.
ls -al /lib/modules/your_new_kernel/weak-updates

drwxr-xr-x. 2 root root 4.0K Sep 16 10:58 .
drwxr-xr-x. 7 root root 4.0K Sep 16 10:58 ..
lrwxrwxrwx  1 root root   54 Sep 16 10:56 splat.ko -> /lib/modules/2.6.32-573.18.1.el6.x86_64/extra/splat.ko
lrwxrwxrwx  1 root root   52 Sep 16 10:56 spl.ko -> /lib/modules/2.6.32-573.18.1.el6.x86_64/extra/spl.ko
lrwxrwxrwx  1 root root   53 Feb 22  2016 zavl.ko -> /lib/modules/2.6.32-573.18.1.el6.x86_64/extra/zavl.ko
lrwxrwxrwx  1 root root   56 Feb 22  2016 zcommon.ko -> /lib/modules/2.6.32-573.18.1.el6.x86_64/extra/zcommon.ko
lrwxrwxrwx  1 root root   52 Sep 16 10:58 zfs.ko -> /lib/modules/2.6.32-573.18.1.el6.x86_64/extra/zfs.ko
lrwxrwxrwx  1 root root   56 Sep 16 10:58 znvpair.ko -> /lib/modules/2.6.32-573.18.1.el6.x86_64/extra/znvpair.ko
lrwxrwxrwx  1 root root   54 Feb 22  2016 zpios.ko -> /lib/modules/2.6.32-573.18.1.el6.x86_64/extra/zpios.ko
lrwxrwxrwx  1 root root   57 Feb 22  2016 zunicode.ko -> /lib/modules/2.6.32-573.18.1.el6.x86_64/extra/zunicode.ko

#If you have no extra modules installed other than ZFS and SPL, you can run the following:
sudo rm -Rf /lib/modules/*/weak-updates/*

#Otherwise just remove the files one by one.

#Now, let's get into the fun part. We will remove them and reinstall them.
#Don't forget to match your version.
sudo dkms remove zfs/ --all
sudo dkms remove spl/ --all
sudo dkms --force install spl/
sudo dkms --force install zfs/

And we will verify the result.

#sudo dkms status
spl,, 3.10.0-514.2.2.el7.x86_64, x86_64: installed
zfs,, 3.10.0-514.2.2.el7.x86_64, x86_64: installed
zfs,, 3.10.0-327.28.3.el7.x86_64, x86_64: installed-weak from 3.10.0-514.2.2.el7.x86_64

The Kernel Version Matters

The kernel version does matter, and I will avoid using version 2.6 or below if you don’t have a professional grade hardware, such as Xeon CPU. Here is my comment:

Linux Kernel (v.2.6)
Linux Kernel (v.3)
9 & 10

Dell Power Edge T100 II
(Intel Xeon E3-1240 V2, 8GB memory, US$250)

Dell Power Edge T320
(Intel Xeon E5-2430, 64GB memory, US$2,000)

Gaming Quality Desktop
(Intel i7-4770, 32GB memory, US$900)

Consumer Grade Desktop
(Intel i3-540, 8GB memory, US$500)

However, it doesn’t mean that you should always use the latest kernel. Remember one thing, always keep a copy of the previous kernel before switching to the latest one. You never know whether ZFS will work with the latest one or not. For example, I have a big trouble to get ZFS working with 2.6.32-573.7.1.el6.x86_64, which is the latest kernel available on CentOS 6.7 (as of Oct 26, 2015). I ended up switching the system to 2.6.32-573.3.1.el6.x86_64 (-1 kernel). So always test the system before making the switch.

The Hard Drive Identifier

Set up your ZFS with the unique, non-changeable hard drive identifier (e.g., /dev/disk/by-id/wwn-0x1234c567890d0aaa). Do not use the generic device id (e.g., /dev/sda). When you reboot the system, the generic device id (/dev/sda) may get changed. This will be a problem to the ZFS.

For example, when RHEL 7 names the hard drive, it will name the hard drives that are attached directly to the motherboard first, these includes USB flash drives, SD card etc. After that, it will name the hard drives that are attached to the PCIe raid card. When you boot the computer with a USB flash drive attached, and if the USB flash drive was not available at the time you set up the ZFS, this small change is good enough to mess up your ZFS.

Here is an example:

History for 'storage':
zpool create -f storage raidz /dev/disk/by-id/wwn-0x5000c500206e46d4 \
                              /dev/disk/by-id/wwn-0x5000c500205eba0d \
                              /dev/disk/by-id/wwn-0x50014ee25a9074e2 \

So far I only noticed this problem with low-end / consumer grade motherboard. However, this is not a problem with FreeBSD because it is smart enough to re-map the old values.

The Stability

For some odd reasons, the ZFS will be unstable or even unavailable when the I/O is heavy:

  pool: storage
 state: DEGRADED
  scan: none requested
status: One or more devices could not be opened.  Sufficient replicas exist for
        the pool to continue functioning in a degraded state.

        NAME                        STATE     READ WRITE CKSUM
        storage                     DEGRADED     0     0     0
          raidz1-0                  DEGRADED     0     0     0
            wwn-0x5000c500206e46d4  ONLINE       0     0     0
            wwn-0x5000c500205eba0d  ONLINE       0     0     0
            wwn-0x50014ee25a9074e2  ONLINE       0     0     0
            wwn-0x50024e9001c19fb2  UNAVAIL      0     0     0

This kind of problem happens mainly with low-end consumer grade computer with older kernel. Once I upgraded the kernel to a newer version, the problem is gone. No hardware change is needed. Again, I’ve never experienced this kind of problem since FreeBSD 9. The only explanation I can think of is the older Linux Kernel does not support ZFS and low-end computer very well.

Load ZFS at Boot

Some Linux variants such as CentOS 7 will not load ZFS at boot (in my case, my kernel is 3.10.0-327.28.3.el7.x86_64). I choose to run the ZFS via cron job. What if the ZFS contains the files that are required by some service, e.g, your database or web server files are on ZFS? You will need to restart those services after loading the ZFS. Here is an example:

sudo nano /etc/crontab

#Example 1: Load all available ZFS pools
@reboot         root    sleep 20; zpool import -a;

#Example 2: Load all ZFS pools first, then restart the Apache, MySQL and NFS services
@reboot         root    sleep 20; zpool import -a; sleep 15; systemctl restart httpd.service && systemctl restart mariadb.service && systemctl restart nfs-server;

Good luck!


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FreeBSD or Linux in 6 Simple Questions

FreeBSD or Linux

FreeBSD or Linux (Ubuntu/RHEL)? This is a very old question. It’s like asking iPhone or Android. There is no short answer. It all depends on your situation. To make things easier, I am going to break it down into six simple questions to help you making decision.

My Background
I have been a FreeBSD user since 2003. My usage on FreeBSD mainly on service-oriented stuffs such as web farm, database clusters and file system etc. In short, I mainly use my FreeBSD system via command line. In 2009, I jumped into Linux world (Ubuntu/RHEL) because of my job. As an advanced user on both Operating Systems (FreeBSD and Ubuntu/RHEL Linux), here is my guide on these two systems.

There is only one FreeBSD, but there are many different variants of Linux. The Linux I mention below refers to two popular distributions: RHEL and Ubuntu.

FreeBSD vs Linux: Q.1 How do you describe yourself?

I am a very demanding person. I like to control everything I manage. –> FreeBSD
I don’t care about how a system is run. I am okay as long as it just works. –> Linux / Ubuntu / RHEL

FreeBSD gives you freedom to control every single thing. One of its coolest feature is the port tree. You can build every application from source using port tree. In Linux world, you usually install applications from pre-built/pre-compiled package (yum, apt-get etc), which may not be exactly what you need.

In RHEL and Ubuntu, HTTP load balancer module does not come with Apache by default. You will need to compile Apache from source. What about in FreeBSD? All you need is to check a box (pretty much like shopping cart) and you are done.

FreeBSD vs Linux: Q.2 Do you prefer Ferrari or Hyundai?

Ferrari / BMW / SLR Camera –> FreeBSD
Toyota / Hyundai / Point and Shoot Camera / Phone Camera –> Linux / Ubuntu / RHEL

The technology uses by FreeBSD such as kernel, file system, architecture etc are way better and more advanced than Linux. It’s like comparing Ferrari and Hyundai (And no, I am not kidding).

Example 1:
I need to run some extreme applications (e.g., DNA Sequence Alignment) which use all available threads and memory. The default settings of memory management in Linux is very poor. Every time I run my application, the system becomes not usable to other users. However, FreeBSD does not have this server problem. In fact, FreeBSD is smart enough not to cause the system to freeze. Of course you can tweak the memory management settings in Linux, or even run the command along with the nice command. However, these settings are just not available out of the box, and most of the time, you learn these tricks after your system has problem.

Example 2:
I installed FreeBSD 11 and RHEL 7 on two identical computers respectively. Both use default settings and enable similar services, such as enabling the SSH server and disable booting to X-Windows (RHEL). I used these two machines for exact the same purposes: SSH tunneling, with exact the same work load (evenly distributed). After using them for a month, I checked the memory usage. With FreeBSD, the available memory is about 800MB (out of 1GB), while there are only 200MB left (out of 1GB) on RHEL. Yes, Linux (at least with RHEL 7) consumes lots of memory.

Example 3:
FreeBSD comes with ZFS (The next generation file system) by default. Although it has been ported to Linux world, it is definitely unstable. We’ve tried to use it in a production environment. One thing that we’ve learned is that the ZFS may stop working after upgrading to a newer Linux kernel. I’ve received countless of email alert about missing the files (ZFS is not working, of course the files are gone) in the middle of the night. I ended up disable the auto update and disable the reboot after the update. Sounds familiar? That’s a feature in Windows, and for some odd reasons, this feature is available in Linux, a server.

Example 4:
LVM+RAID is an the most advanced storage method in Linux world. Unfortunately it does not do what it promises, i.e., you may lose your data if a hard drive is failure, even if you follow its directions to detach your failed hard drive correctly. Not to mention that the data will get corrupted if the power is down (which has been taken care of in ZFS, available in FreeBSD).

That’s why I prefer ZFS over LVM+RAID here: Building a Super Large and Reliable File Server with Mixed Size of Harddisks. It solves my problem (yes, even the power is failed during writing to the disk, my data is still safe!)

FreeBSD vs Linux: Q.3 Do you have lots of free time?

Yes: FreeBSD
No: Linux / Ubuntu / RHEL

Making a production-ready system using FreeBSD can take you days to weeks if you are not an experienced FreeBSD user, while everything works out of the box in Linux. Sometimes, the new upgrade from the port tree can drive you nut, such as package conflicts etc. However, working with Linux is a leisure thing.

Installing Apache + MySQL + PHP from FreeBSD port (compiling the source) can take at least half day on a computer with a dual core CPU (AMD Athlon 64 X2 Dual Core Processor 4600+), while it takes less than 30 minutes on Linux. That’s because you need to compile the code from source in FreeBSD, while you simply download the packages and extract them in Linux. The time difference is huge.

FreeBSD vs Linux: Q.4 Do you prefer simplicity or complexity?

Adding sugar into water –> FreeBSD
Taking sugar away from soda–> Linux / Ubuntu / RHEL

FreeBSD is a very very simple system. Think of it as a bare-bone system that comes with no junk. By default, it comes with no graphic user interface, no unnecessary application. It likes a pure distilled water.

In Ubuntu / RHEL, everything is configured and ready to use. It comes with very attractive, beautiful graphical user interface. Everything just works out of the box, no tuning or tweaking is required. However, it also comes with lots of junk such as Ubuntu One (For Ubuntu Cloud service), SELinux (A program developed by Red Hat where many people don’t use it), etc. It makes your system very bulky and increase the computation resource consumption.

FreeBSD vs Linux: Q.5 Are you going to use the computer as desktop?

Yes: Linux / Ubuntu / RHEL
No: FreeBSD

Setting up a desktop-ready system on FreeBSD can take a long time. Main reason is the drivers availability. A lot of hardware such as graphic cards, audio cards or webcams are not supported in FreeBSD natively. If you want to get them working as good as on Windows / Ubuntu, you will need to get the driver first (if available), build it (which may gives error during compiling the codes), and recompile the kernel to make it supports the new driver, which can take few days if you are not experienced with FreeBSD and debugging driver.

On the other side, the Linux driver community is very strong and well developed. Usually they develop drivers for most popular hardware.


#1: My Logitech Orbit MP webcam (Pan / Tilt / Zoom) is not working on FreeBSD but works like a charm on Linux.

#2: Some vendors such as Highpoint may stop developing drivers for their products for newest version of FreeBSD.

FreeBSD vs Linux: Q.6 Do you need to blame someone when something goes wrong?

Yes: Linux / Ubuntu / RHEL
No: FreeBSD


When something goes wrong, you can blame Linux and you can’t blame FreeBSD.

FreeBSD is a community driven operating system, while some Linux distributions such as Ubuntu and RHEL are backed by commercial vendors. In the world of FreeBSD, it is not uncommon that some unskilled developers introduce bugs, troubles to the rest of the world. Yes, we know it is free, so we can’t complaint about it. However, as an IT administrator, you will need to use your judgement to judge whether those new stuffs are safe to use or not. In short, that will increase your workload. It’s more like Windows update. How many people actually read the change log before hitting the update button?

In Linux (e.g., RHEL), that’s a whole different story. Every patches, new updates have been screened by the vendor before hitting to public. So you can trust them in some degree. And the key thing is, you can blame them when something goes wrong.


In short, use FreeBSD for your personal purpose while Linux for your work. Be the top 5%, not the bottom 95%.

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CentOS 6: No Networking Connections After Upgrade

Today I reboot my CentOS 6 server, and I realized that the network connection was lost after the upgrade. To be exact, it seems that the problem was caused by the new kernel: 2.6.32-573.1.1.el6.x86_64. It modified the network settings of the server with manual settings (server with DHCP is not affected). Here is how I fix the problem (You will need physical access to the server):

I have noticed that the adapter profile has been modified to something that doesn’t make scenes. If you compare the network settings, you will notice the following differences:

#Before the upgrade
#cat /etc/sysconfig/network-scripts/ifcfg-em1  
#After the upgrade
#cat /etc/sysconfig/network-scripts/ifcfg-em1  

So I simply make the modifications to the adapter settings and restart the network service, i.e.,

sudo service network restart

And the network connection is back!

That’s it! Hope this tutorial saves you from heart attack.


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[FreeBSD]Upgrade PHP 5.5 to 5.6

It is not easy to upgrade PHP 5.5 to 5.6 in FreeBSD. Without proper preparation, the upgrade process may drive you nut. Before you decide to get your hands wet, here are what I recommend you to do:

  1. Back up your files
  2. Test your website in a PHP 5.6 environment on a different server. It is because PHP 5.6 has introduced some backward incompatibilities. Some of the codes written in the prior versions may introduce run time error. See here for more information.
  3. Schedule a down time. Depending on your CPU speed / typing speed / trouble-shooting skill, it may take you an hour.


I am assuming that you use PHP for web purposes (rather than command line / CLI only), and I am assuming that you are using PHP with Apache. Here are the ports you will need to touch:

  • Apache: /usr/ports/www/apache22 or /usr/ports/www/apache24
  • Apache-PHP: /usr/ports/www/mod_php56
  • PHP: /usr/ports/lang/php56
  • PHP Extensions: /usr/ports/lang/php56-extensions

1. Remove the old PHP and extensions

cd /usr/ports/lang/php55
sudo make deinstall clean

cd /usr/ports/lang/php55-extensions
sudo make deinstall clean

2. Install PHP 5.6

cd /usr/ports/lang/php56

#Don't forget enable ZTS if you have threaded Apache.
sudo make install clean

3. Install PHP 5.6 Extensions

cd /usr/ports/lang/php56-extensions
sudo make install clean

4. Test PHP and its extensions

php -v
php -m

Clean up the error by removing the duplicated entries in:

5. Rebuild the Apache-PHP Bridge

cd /usr/ports/www/mod_php55
sudo make deinstall clean

cd /usr/ports/www/mod_php56
#Don't forget enable ZTS if you have threaded Apache.
sudo make install clean

6. Restart Apache

sudo /usr/local/etc/rc.d/apache restart

7. Test PHP using phpinfo

Create a code called test.php to display phpinfo. Verify that everything is okay.

< ?php

8. Reinstall Apache (optional)

If you experience any problem, try to reinstall the following ports:

Apache: /usr/ports/www/apache22 or /usr/ports/www/apache24
Apache-PHP: /usr/ports/www/mod_php56

That’s it! Enjoy the new PHP!


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MySQL Random Error: ERROR 2013 (HY000): Lost connection to MySQL server at ‘reading authorization packet’, system error: 0

Recently, I am experiencing a weird error when connecting to a MySQL server remotely:

ERROR 2013 (HY000): Lost connection to MySQL server at 'reading authorization packet', system error: 0

Basically, this error is similar to the busy tone when you are making calls. The key thing is, it happens randomly. Sometimes the connection is okay, sometimes it takes any where from 0.01 to 30 seconds to establish a connection. Sometimes it gets time-out.

Long story short. Continue to read this article if you have met the following conditions:

  • You try to connect to a MySQL server remotely, i.e., not localhost(
  • It happens randomly. It can take anywhere from 0.01 seconds to 30 seconds to establish a connection. Sometimes it fails.
  • You connect the server using IP address, i.e., it has nothing to do with the domain name, or skip-name-resolve in my.cnf
  • You have included the client IP address in /etc/hosts.allow.

The key thing is: Random.

You probably have scratched your head for few hours (or days), gone through tons of useless suggestions on Google/Stackoverflow/Serverfault etc, and the problem still exists. Oh well, at least this has been happened on me in the past 24 hours.

Before we discussed the problem, let’s try to reproduce the problem:

#In the client computer, we try to connect to 
#the MySQL database remotely and run a simply command:
time mysql -u root -pPASSWORD -h IP_ADDRESS -e "show databases;"

#Case 1: Everything is okay
real    0m0.001s
user    0m0.001s
sys     0m0.001s

#Case 2: it takes 20 seconds to establish a connection. 
#That's not right.
real    0m20.001s
user    0m0.003s
sys     0m0.003s

#Case 3: Cannot even make the connection.
ERROR 2013 (HY000): Lost connection to MySQL server at 'reading authorization packet', system error: 0
real    0m49.617s
user    0m0.003s
sys     0m0.003s

If you also observe a similar symptoms, I can tell you that the problem may not be related to MySQL server or MySQL settings. I recommend you to check your network traffic. Here are my suggestions:

#Check which process is running.

#Or you can check which process is running by the web server user
#In my case, apache is the web server user
ps -u apache

#Or you can check the current traffic using nload
nload -u M

If you are lucky, you may notice that there is a huge network traffic going on. The traffic is the main problem that cause the problem. Try to kill that process or perform a reboot.

Let’s take my case an example. I noticed a weird process running by the apache user:

ps -u apache

  PID TTY          TIME CMD
 8112 ?        00:00:09 httpd
 8113 ?        00:00:08 httpd
 8334 ?        00:00:08 httpd
 8796 ?        00:00:06 httpd
 8802 ?        00:00:07 httpd
 8891 ?        00:00:07 something (This is a malware)

After I kill that process, everything is back to normal again.


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ZFS Performance: Mirror VS RAIDZ VS RAIDZ2 vs RAIDZ3 vs Striped

I always wanted to find out the performance difference among different ZFS types, such as mirror, RAIDZ, RAIDZ2, RAIDZ3, Striped, two RAIDZ vdevs vs one RAIDZ2 vdev etc. So I decide to create an experiment to test these ZFS types. Before we talk about the test result, let’s go over some background information, such as the details of each design and the hardware information.


Here is a machine I used for experiment. It is a consumer grade desktop computer manufactured back in 2014:

CPU: Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz / quard cores / 8 threads
OS: CentOS Linux release 7.3.1611 (Core)
Kernel: Linux 3.10.0-514.6.1.el7.x86_64 #1 SMP Wed Jan 18 13:06:36 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
Memory: 20 GB (2GB x 4)
Hard drives: 5 TB x 8 
(Every hard drive is 4k sectors, non-SSD, consumer grade, connected via a PCI-e x 16 raid card with SAS interface)
System Settings: Everything is system default. Nothing has been done to the kernel configuration.

Also, I tried to keep each test simple. Therefore I didn’t do anything special:

zpool create -f myzpool (different settings go here...)
zfs create myzpool/data

To optimize the I/O performance, the block size of the zpool is based on the physical sector of the hard drive. In my case, all of the hard drives have 4k (4096 bytes) sectors, which is translated to 2^12, therefore, the ashift value of the zpool is 12.

zdb | grep ashift
ashift: 12

To measure the write performance, I first generate a zero based file with the size of 41GB and output to the zpool directly. To measure the read performance, I read the file and output to /dev/null. Notice that the file size is very large (41GB) such that it does not fit in the arc cache memory (50% of the system memory, i.e., 10GB). Notice that the block size is the physical sector of the hard drive.

One of the readers asked me why I use a large file instead of many small files. There are few reasons:

  • It is very easy to stress test / saturate the bandwidth (connection in between the hard drives, network etc) when working with large file.
  • The results of testing large files is more consistent.
#To test the write performance:
dd if=/dev/zero of=/myzpool/data/file.out bs=4096 count=10000000

#To test the read performance:
dd if=/myzpool/data/file.out of=/dev/null bs=4096

FYI, if the block size is not specified, the result can be very different:

#Using default block size:
dd if=/myzpool/data/file.out of=/dev/null
40960000000 bytes (41 GB) copied, 163.046 s, 251 MB/s

#Using native block size:
dd if=/myzpool/data/file.out of=/dev/null bs=4096
40960000000 bytes (41 GB) copied, 58.111 s, 705 MB/s

After each test, I destroyed the zpool and created a different one. This ensures that the environment factors (such as hardware and OS) stay the same. Here is the test result. If you want to learn more about each design, such as the exact command I used for each test, the corresponding material will be available in the later section.

Notice that I used eight 5TiB hard drives (Total: 40TiB) in this test. Typically hard drive of 5TiB of can hold about 4.5 TB of data, that’s around 86%-90% of the advertised number, depending on which OS you are using. For example, if we use the striped design, which is the maximum possible storage capacity in ZFS, the usable space will be 8 x 5TiB x 90% = 36TB. Therefore, the following percentages will be based on 36TB rather than 40TiB.

You may notice that I use 10 disks in each diagram, while I use only 8 disks in the article here. That’s because the diagram was from my first edit. At that time I used a relative old machine, which may not reflect the modern ZFS design. The hardware and the test methods I used in the second edit is better, although both edits draw the same conclusion.

Test Result

(Sorted by speed)

ZFS Type
(Click to see details)
Write Speed (MB/s)
Time Spent on Writing a 41GB File
Read Speed (MB/s)
Time Spent on Reading a 41GB File
Storage Capacity (Max: 36TB)
# of Disks Used On Data Parity
Disk Arrangement

36TB (100%)
Striped (8)

26TB (72%)
RAIDZ (4) x 2

25TB (69%)
RAIDZ2 (8)

30TB (83%)

21TB (58%)
RAIDZ3 (8)

18TB (50%)
Mirror (2) x 4

18TB (50%)
RAIDZ(2) x 2


In this design, we use all disks to store data (i.e., zero data protection), which max out our total usable spaces to 36 TB.

zpool create -f myzpool hd1 hd2 \
                        hd3 hd4 \
                        hd5 hd6 \
                        hd7 hd8

#df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
myzpool         36T      0K     36T       0%  /myzpool 

#zpool status -v
        NAME        STATE     READ WRITE CKSUM
        myzpool     ONLINE       0     0     0
          hd1       ONLINE       0     0     0
          hd2       ONLINE       0     0     0
          hd3       ONLINE       0     0     0
          hd4       ONLINE       0     0     0
          hd5       ONLINE       0     0     0
          hd6       ONLINE       0     0     0
          hd7       ONLINE       0     0     0
          hd8       ONLINE       0     0     0

And here is the test result:

#Write Test
dd if=/myzpool/data/file.out of=/dev/null bs=4096
40960000000 bytes (41 GB) copied, 58.111 s, 705 MB/s

#Read Test
dd if=/myzpool/data/file.out of=/dev/null bs=4096
40960000000 bytes (41 GB) copied, 59.6386 s, 687 MB/s


In this design, we split the data into two groups. In each group, we store the data in a RAIDZ1 structure. This is similar to RAIDZ2 in terms of data protection, except that this design supports up to one failure disk in each group (local scale), while RAIDZ2 allows ANY two failure disks overall (global scale). Since we use two disks for parity purpose, the usable space drops from 36TB to 26TB.

zpool create -f myzpool raidz hd1 hd2 hd3 hd4 \
                        raidz hd5 hd6 hd7 hd8

#df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
myzpool         26T      0K     26T       0%  /myzpool 

#zpool status -v
        NAME        STATE     READ WRITE CKSUM
        myzpool     ONLINE       0     0     0
          raidz1-0  ONLINE       0     0     0
            hd1     ONLINE       0     0     0
            hd2     ONLINE       0     0     0
            hd3     ONLINE       0     0     0
            hd4     ONLINE       0     0     0
          raidz1-1  ONLINE       0     0     0
            hd5     ONLINE       0     0     0
            hd6     ONLINE       0     0     0
            hd7     ONLINE       0     0     0
            hd8     ONLINE       0     0     0

And here is the test result:

#Write Test
dd if=/dev/zero of=/storage/data/file.out bs=4096 count=10000000
40960000000 bytes (41 GB) copied, 61.1401 s, 670 MB/s

#Read Test
dd if=/storage/data/file.out of=/dev/null bs=4096
40960000000 bytes (41 GB) copied, 60.2457 s, 680 MB/s


In this design, we use two disks for data protection. This allow up to two disks fail without losing any data. The usable space will drop from 36TB to 25TB.

zpool create -f myzpool raidz2 hd1 hd2 hd3 hd4 \
                               hd5 hd6 hd7 hd8

#df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
myzpool         25T     31K     25T       0%  /myzpool 

#zpool status -v
        NAME        STATE     READ WRITE CKSUM
        myzpool     ONLINE       0     0     0
          raidz2-0  ONLINE       0     0     0
            hd1     ONLINE       0     0     0
            hd2     ONLINE       0     0     0
            hd3     ONLINE       0     0     0
            hd4     ONLINE       0     0     0
            hd5     ONLINE       0     0     0
            hd6     ONLINE       0     0     0
            hd7     ONLINE       0     0     0
            hd8     ONLINE       0     0     0

And here is the test result:

#Write Test
dd if=/dev/zero of=/storage/data/file.out bs=4096 count=10000000
40960000000 bytes (41 GB) copied, 67.3897 s, 608 MB/s

#Read Test
dd if=/storage/data/file.out of=/dev/null bs=4096
40960000000 bytes (41 GB) copied, 60.8205 s, 673 MB/s


In this design, we use one disk for data protection. This allow up to one disk fails without losing any data. The usable space will drop from 36TB to 30TB.

zpool create -f myzpool raidz hd1 hd2 hd3 hd4 \
                              hd5 hd6 hd7 hd8

#df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
myzpool         30T      0K     30T       0%  /myzpool 

#zpool status -v
        NAME        STATE     READ WRITE CKSUM
        myzpool     ONLINE       0     0     0
          raidz1-0  ONLINE       0     0     0
            hd1     ONLINE       0     0     0
            hd2     ONLINE       0     0     0
            hd3     ONLINE       0     0     0
            hd4     ONLINE       0     0     0
            hd5     ONLINE       0     0     0
            hd6     ONLINE       0     0     0
            hd7     ONLINE       0     0     0
            hd8     ONLINE       0     0     0

And here is the test result:

#Write Test
dd if=/dev/zero of=/storage/data/file.out bs=4096 count=10000000
40960000000 bytes (41 GB) copied, 67.8107 s, 604 MB/s

#Read Test
dd if=/storage/data/file.out of=/dev/null bs=4096
40960000000 bytes (41 GB) copied, 64.8782 s, 631 MB/s


In this design, we use three disks for data protection. This allow up to three disks fail without losing any data. The usable space will drop from 36TB to 21TB.

zpool create -f myzpool raidz3 hd1 hd2 hd3 hd4 \
                               hd5 hd6 hd7 hd8

#df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
myzpool         21T     31K     21T     0%    /myzpool 

#zpool status -v
        NAME        STATE     READ WRITE CKSUM
        myzpool     ONLINE       0     0     0
          raidz3-0  ONLINE       0     0     0
            hd1     ONLINE       0     0     0
            hd2     ONLINE       0     0     0
            hd3     ONLINE       0     0     0
            hd4     ONLINE       0     0     0
            hd5     ONLINE       0     0     0
            hd6     ONLINE       0     0     0
            hd7     ONLINE       0     0     0
            hd8     ONLINE       0     0     0

And here is the test result:

#Write Test
dd if=/dev/zero of=/storage/data/file.out bs=4096 count=10000000
40960000000 bytes (41 GB) copied, 77.549 s, 528 MB/s

#Read Test
dd if=/storage/data/file.out of=/dev/null bs=4096
40960000000 bytes (41 GB) copied, 70.9604 s, 577 MB/s


In this design, we use half of our disks for data protection, which makes our total usable spaces drop from 36 TB to 18 TB.

zpool create -f myzpool mirror hd1 hd2 \
                        mirror hd3 hd4 \
                        mirror hd5 hd6 \
                        mirror hd7 hd8

#df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
myzpool         18T     31K     18T       0%  /myzpool 

#zpool status -v
        NAME        STATE     READ WRITE CKSUM
        myzpool     ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            hd1     ONLINE       0     0     0
            hd2     ONLINE       0     0     0
          mirror-1  ONLINE       0     0     0
            hd3     ONLINE       0     0     0
            hd4     ONLINE       0     0     0
          mirror-2  ONLINE       0     0     0
            hd5     ONLINE       0     0     0
            hd6     ONLINE       0     0     0
          mirror-3  ONLINE       0     0     0
            hd7     ONLINE       0     0     0
            hd8     ONLINE       0     0     0

And here is the test result:

#Write Test
dd if=/dev/zero of=/storage/data/file.out bs=4096 count=10000000
40960000000 bytes (41 GB) copied, 86.6451 s, 473 MB/s

#Read Test
dd if=/storage/data/file.out of=/dev/null bs=4096
40960000000 bytes (41 GB) copied, 68.4477 s, 598 MB/s

RAIDZ2 x 2

In this design, we split the data into two groups. In each group, we store the data in a RAIDZ2 structure. Since we use two disks for parity purpose, the usable space drops from 36TB to 18TB.

zpool create -f myzpool raidz2 hd1 hd2 hd3 hd4 \
                        raidz2 hd5 hd6 hd7 hd8

#df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
myzpool         18T      0K     18T       0%  /myzpool 

#zpool status -v
        NAME        STATE     READ WRITE CKSUM
        myzpool     ONLINE       0     0     0
          raidz2-0  ONLINE       0     0     0
            hd1     ONLINE       0     0     0
            hd2     ONLINE       0     0     0
            hd3     ONLINE       0     0     0
            hd4     ONLINE       0     0     0
          raidz2-1  ONLINE       0     0     0
            hd5     ONLINE       0     0     0
            hd6     ONLINE       0     0     0
            hd7     ONLINE       0     0     0
            hd8     ONLINE       0     0     0

And here is the test result:

#Write Test
dd if=/dev/zero of=/storage/data/file.out bs=4096 count=10000000
40960000000 bytes (41 GB) copied, 98.9698 s, 414 MB/s

#Read Test
dd if=/storage/data/file.out of=/dev/null bs=4096
40960000000 bytes (41 GB) copied, 92.963 s, 441 MB/s


I am not surprised that the striped layout offers the fastest writing speed and maximum storage space. The only drawback is zero data protection. Unless you mirror the data at the server level (e.g., Hadoop), or the data is not important, otherwise I won’t recommend you to use this design.

Personally I recommend to go with Striped RAIDZ, i.e., we try to make multiple RAIDZ vdev, and each vdev has no more than 5 disks. In theory, ZFS recommends the number of disks in each vdev is no more than 8 to 9 disks. Based on my experience, ZFS will slow down when it has about 30% free space left if we have too many disks in one single vdev.

So which design you should use? Here is my recommendation:

#Do you care your data?
No: Go with striped.
Yes: See below:

#How many disks do you have?
1:     ZFS is not for you.
2:     Mirror
3-5:   RAIDZ1
6-10   RAIDZ1 x 2
10-15: RAIDZ1 x 3
16-20: RAIDZ1 x 4

And yes, you can pretty much forget about RAIDZ2, RAIDZ3 and mirror if you need speed and data protection together.

So, you may ask a question, what should I do if there are more than one hard drive fail? The answer is: You need to keep an eye on the health of your ZFS pool every day. I have been managing over 60 servers since 2009, and I’ve used only RAIDZ1 with my consumer level harddrives (most of them actually was taken from the external harddrives). So far I don’t have any data lost.

sudo zpool status -v


sudo zpool status -v | grep 'state: ONLINE'

Simply write a program to get the result from this command, and send yourself an email if there is anything go wrong. You can include the program in your cron job and have it run daily or hourly. This is my version:


result=`sudo zpool status -x`

if [[ $result != 'all pools are healthy' ]]; then
        echo "Something is wrong."
        #Do something here such as send an email, such as sending an email via HTTP...
        /usr/bin/wget "" -O /dev/null > /dev/null
        exit 1;

Enjoy ZFS.


Our sponsors:

This rsync lacks old-style –compress due to its external zlib. Try -zz.

When I tried to run rsync (3.1.1) on my FreeBSD box today, the following message caught my attention:

#rsync -avzr --rsh="ssh -c arcfour" --delete --compress-level=9  sourcemachine:/source/  /target/

This rsync lacks old-style --compress due to its external zlib.  Try -zz.
Continuing without compression.

Basically, rsync suggests that you should use the -zz option instead of the old style option. So I gave it a try, and of course, I got something like this:

#rsync -avr --rsh="ssh -c arcfour" --delete -zz  sourcemachine:/source/  /target/

#My target machine is CentOS / rsync (3.0.6)
rsync: on remote machine: --new-compress: unknown option
rsync error: syntax or usage error (code 1) at main.c(1422) [server=3.0.6]
rsync: connection unexpectedly closed (0 bytes received so far) [Receiver]
rsync error: error in rsync protocol data stream (code 12) at io.c(226) [Receiver=3.1.1]

The reason why we have all of these messy things because there is a bug in rsync 3.1.1. (rsync –version) To solve this problem, you will need to rebuild the rsync by skipping the ZLIB_BASE option:

cd /usr/ports/net/rsync
sudo make config

#Uncheck the ZLIB_BASE option

sudo make reinstall clean

Now the rsync should be happy.


Our sponsors:

How to Upgrade Google Chrome to 64-bit

Recently Google finally made the Google Chrome 64-bit available in Windows. Since Google Chrome is pretty much maintenance free, i.e., it updates automatically and I don’t need to take care of it. I was wondering whether it will upgrade to 64-bit itself. Unfortunately, I couldn’t find such information on the web.

Long story short. If you are using Google Chrome 32-bit, it WILL NOT upgrade itself to 64-bit. You will need to download Google Chrome and install it again.

Here are some information if you like to find out whether your Google Chrome is 32-bit or 64-bit. First, open the Windows Task Manager, or simply type “taskmgr” in command line:

Open Google Chrome, and you should see several processes like chrome.exe. If it is ended with *32 like below, then the Google Chrome is running on 32-bit.

Once you re-install Google Chrome, check the status using Windows Task Manager again. This time you should see a different picture:

Google Chrome 64-Bit

If the name of the process is not ended with *32, then the Google Chrome is running on 64-bit.

Is Google Chrome 64-bit faster? Technically it is.


Our sponsors:

How to Improve ZFS performance

First Edition: March 14, 2010
Last Edited: January 18, 2021


In this tutorial, I will show you how to improve the performance of your ZFS using the affordable consumer-grade hardware (e.g., Gigabit network card, standard SATA non-SSD hard drives, consumer-grade motherboard etc.).

Many people found a problem on their ZFS system. The speed is slow! It is slow to read or write files to the system. In this article, I am going to show you some tips on improving the speed of your ZFS file system.

Notice that this article was originally based on ZFS on FreeBSD. Although most concepts can be applied to Linux, you may want to check out these two articles: ZFS: Linux VS FreeBSD and ZFS On Linux Emergency Recovery Guide. I always go back to the second article to rescure my ZFS data after rebooting my Linux server to a newer kernel.

This article is about how to build a single node ZFS server. If you are interested in implementing multiple-nodes ZFS system / ZFS clusters, please check here for details.

Table of Content
  1. A Good 64-bit CPU + Lots of Memory
  2. Tweaking the Boot Loader
  3. Use Disks with the Same Specifications
  4. Use a Powerful Power Supply
  5. Enable Compression
  6. Identify the Bottleneck
  7. Keep Your ZFS up to Date
  8. Understand How the ZFS Caching Works
  9. More Disk = Faster
  10. Use a Combination of Strip and RAIDZ If Speed Is Your First Concern.
  11. Distribute Your Free Space Evenly (How to Rebalance ZFS)
  12. Make Your Zpool Expandable
  13. Backup the Data on a Different Machine, Not on the Same Zpool
  14. Rsync or ZFS Send?
  15. Did You Enable Dedup? Disable It!
  16. Reinstall Your Old System
  17. Connect Your Disks via High Speed Interface
  18. Do Not Use up All Spaces
  19. Use AHCI, Not IDE
  20. Refresh Your Zpool
  21. Great Performance Settings
  22. My Settings

Improve ZFS Performance: Step 1

A Good 64-bit CPU + Lots of Memory

Traditionally, we are told to use a less powerful computer for a file/data server. That’s not true for ZFS. ZFS is more than a file system. It uses a lot of resources to improve the performance of the input/output, such as compressing data on the fly. For example, suppose you need to write a 1GB file. Without enabling the compression, the system will write the entire 1GB file on the disk. With the compression being enabled, the CPU will compress the data first, and write the data on the disk after that. Since the compressed data is smaller, it takes shorter time to write to the disk, which results a higher writing speed. The same thing can be applied for reading. ZFS can cache the file for you in the memory, it will result a higher reading speed.

That’s why a 64-bit CPU and higher amount of memory is recommended. I recommended at least a Quad Core CPU with 4GB of memory (Personally I use Xeon and i7, with at least 20GB of memory).

Please make sure that the memory modules should have the same frequencies/speed. If you mix them with different speed, try to group the memories with same speed together., e.g., Channel 1 and Channel 2: 1333 MHz, Channel 3 and Channel 4: 1600 MHz.

Let’s do a test. Suppose I am going to create a 10GB file with all zero. Let’s see how long does it take to write on the disk:

#CPU: i7 920 (A 4 cores/8 threads CPU from 2009) + 24GB Memory + FreeBSD 9.3 64-bit
#dd if=/dev/zero of=./file.out bs=1M count=10k
10737418240 bytes transferred in 6.138918 secs (1749073364 bytes/sec)

That’s 1.6GB/s! Why is it so fast? That’s because it is a zero based file. After the compression, a compressed 10GB file may result in few bytes only. Since the performance of the compression is highly depended on the CPU, that’s why a fast CPU matters.

Now, let’s do the same thing on a not-so-fast CPU:

#CPU: AMD 4600 (2 cores) + 5GB Memory + FreeBSD 9.3 64-bit
#dd if=/dev/zero of=./file.out bs=1M count=10k
10737418240 bytes transferred in 23.672373 secs (453584362 bytes/sec)

That’s 434MB/s only. See the difference?

Improve ZFS Performance: Step 2

Tweaking the Boot Loader Parameters

Update: This section was written based on FreeBSD 8. As of today (February 12, 2017), the latest version is FreeBSD 11. I noticed that my FreeBSD/ZFS works very stable even without any tweaking! In the other words, you may skip this section if you are using FreeBSD 8.2 or later.

Many people complain about ZFS for its stability issues, such as kernel panic, reboot randomly, crash when copying large files (> 2GB) at full speed etc. It may have something to do with the boot loader settings. By default, ZFS will not work smoothly without tweaking the system parameters system. Even FreeBSD (9.1 or earlier) claims that no tweaking is necessary for 64-bit system, my FreeBSD server crashes very often when writing large files to the pool. After trial and error for many times, I figure out few equations. You can tweak your boot loader (/boot/loader.conf) using the following parameters. Notice that I only tested the following on FreeBSD. Please let me know whether the following tweaks work on other operating systems.

If you experiences kernel panic, crash or something similar, it could be the hardware problem, such as memory. I encourage to test all memory modules by using Memtest86+ first. I wish someone told me about this few years ago. That would make my life a lot easier.

Warning: Make sure that you save a copy before doing anything to the boot loader. Also, if you experience anything unusual, please remove your changes and go back to the original settings.

#Assuming 8GB of memory

#If Ram = 4GB, set the value to 512M
#If Ram = 8GB, set the value to 1024M

#Ram x 0.5 - 512 MB

#Ram x 2

#Ram x 1.5

#The following were copied from FreeBSD ZFS Tuning Guide

# Disable ZFS prefetching
# Increases overall speed of ZFS, but when disk flushing/writes occur,
# system is less responsive (due to extreme disk I/O).
# NOTE: Systems with 4 GB of RAM or more have prefetch enabled by default.

# Decrease ZFS txg timeout value from 30 (default) to 5 seconds.  This
# should increase throughput and decrease the "bursty" stalls that
# happen during immense I/O with ZFS.
# default in FreeBSD since ZFS v28

# Increase number of vnodes; we've seen vfs.numvnodes reach 115,000
# at times.  Default max is a little over 200,000.  Playing it safe...
# If numvnodes reaches maxvnode performance substantially decreases.

# Set TXG write limit to a lower threshold.  This helps "level out"
# the throughput rate (see "zpool iostat").  A value of 256MB works well
# for systems with 4 GB of RAM, while 1 GB works well for us w/ 8 GB on
# disks which have 64 MB cache.

# NOTE: in v27 or below , this tunable is called 'vfs.zfs.txg.write_limit_override'.

Don’t forget to reboot your system after making any changes. After changing to the new settings, the writing speed improves from 60MB/s to 80MB/s, sometimes it even goes above 110MB/s! That’s a 33% improvement!

By the way, if you found that the system still crashes often, the problem could be an uncleaned file system.

After a system crashes, it may cause the file links to be broken (e.g., the system sees the file tag, but unable to locate the files). Usually FreeBSD will automatically run fsck after the crash. However, it will not fix the problem for you. In fact, there is no way to clean up the file system when the system is running (because the partition is mounted). The only way to clean up the file system is by entering the Single User Mode (a reboot is required).

After you enter the single user mode, make sure that each partition is cleaned. For example, here is my df result:

Filesystem    Size    Used   Avail Capacity  Mounted on
/dev/ad8s1a   989M    418M    491M    46%    /          
devfs         1.0k    1.0k      0B   100%    /dev       
/dev/ad8s1e   989M     23M    887M     3%    /tmp       
/dev/ad8s1f   159G     11G    134G     8%    /usr       
/dev/ad8s1d    15G    1.9G     12G    13%    /var        

Try running the following commands:

#-y: All yes
#-f: Force
fsck -y -f /dev/ad8s1a
fsck -y -f /dev/ad8s1d
fsck -y -f /dev/ad8s1e
fsck -y -f /dev/ad8s1f

These command will clean up the affected file systems.

After the clean-up is done, type reboot and let the system to boot to the normal mode.

Improve ZFS Performance: Step 3

Use disks with the same specifications

A lot of people may not realize the importance of using exact the same hardware. Mixing different disks of different models/manufacturers can bring performance penalty. For example, if you are mixing a slow disk (e.g., green disk) and a fast disk(e.g., performance disk) in the same virtual device (vdev), the overall speed will depend on the slowest disk. Also, different hard drives may have different sector size. For example, Western Digital releases a hard drive with 4k sector, while the older models use 512 byte. Mixing hard drives with different sectors can bring performance penalty too. Here is a quick way to check the model of your hard drive:

sudo smartctl -a /dev/sda | grep 'Sector Size'

Here is an example output:

Sector Sizes:     512 bytes logical, 4096 bytes physical

If you don’t have enough budget to replace all disks with the same specifications, try to group the disks with similar specifications in the same vdev, e.g.,

sudo zpool create myzpool 
   raidz /dev/slow_disk1 /dev/slow_disk2 /dev/slow_disk3 ...
   raidz /dev/fast_disk1 /dev/fast_disk2 /dev/fast_disk3 ...

Suppose I have a group of hard drives with 4k sector (i.e., 4k = 4096 bytes = 2^12 bytes), which translate to ashift=12:

sudo zpool create myzpool -o ashift=12 raidz1 /dev/disk1 /dev/disk2 ...

You can also verify the ashift value using this command:

sudo zdb | grep shift

Let’s say we have a group of hard drives with 4k sector, we create two ZFS, one with ashift=12, and another one with the default value (ashift=9), here is the difference:

#Using default block size:
dd if=/myzpool/data/file.out of=/dev/null
40960000000 bytes (41 GB) copied, 163.046 s, 251 MB/s

#Using native block size:
dd if=/myzpool/data/file.out of=/dev/null
40960000000 bytes (41 GB) copied, 58.111 s, 705 MB/s

Improve ZFS Performance: Step 4

Use a Powerful Power Supply

I recently built a Linux-based ZFS file system with 12 hard disks. For some reasons, it was pretty unstable. When I tried filling the pool with 12TB of data, the ZFS system crashed randomly. When I rebooted the machine, the error was gone. However, when I resumed the copy process again, the error happened on a different disk. In short, the disks failed randomly. When I checked the dmesg, I found something like the following. Since I didn’t have the exact copy, I grabbed something similar from the web:

[  412.575724] ata10.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6
[  412.576452] ata10.00: BMDMA stat 0x64
[  412.577201] ata10.00: failed command: WRITE DMA EXT
[  412.577897] ata10.00: cmd 35/00:08:97:19:e4/00:00:18:00:00/e0 tag 0 dma 4096 out
[  412.577901]          res 51/84:01:9e:19:e4/84:00:18:00:00/e0 Emask 0x10 (ATA bus error)
[  412.579294] ata10.00: status: { DRDY ERR }
[  412.579996] ata10.00: error: { ICRC ABRT }
[  412.580724] ata10: soft resetting link
[  412.844876] ata10.00: configured for UDMA/133
[  412.844899] ata10: EH complete
[  980.304788] ata10.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6
[  980.305311] ata10.00: BMDMA stat 0x64
[  980.305817] ata10.00: failed command: WRITE DMA EXT
[  980.306351] ata10.00: cmd 35/00:08:c7:00:ce/00:00:18:00:00/e0 tag 0 dma 4096 out
[  980.306354]          res 51/84:01:ce:00:ce/84:00:18:00:00/e0 Emask 0x10 (ATA bus error)
[  980.307425] ata10.00: status: { DRDY ERR }
[  980.307948] ata10.00: error: { ICRC ABRT }
[  980.308529] ata10: soft resetting link
[  980.572523] ata10.00: configured for UDMA/133

Basically, this message means the disk was failed during writing the data. Initially I thought it could be the SMART/bad sectors. However, since the problem happened randomly on random disks, I think the problem could be something else. I have tried to replacing the SATA cable, power cable etc. None of them worked. Finally, I upgraded my power supply (450W to 600W), and the error was gone.

FYI, here is the specs of my affected system. Notice that I didn’t use any component that required high-power such as graphic card etc.

  • CPU: Intel Q6600
  • Standard motherboard
  • PCI RAID Controller Card with 4 SATA ports
  • WD Green Drive x 12
  • CPU fan x 1
  • 12″ case fan x 3

And yes, you will need a 600W power supply for such a simple system. Also, another thing worth to check is the power cable. Sometimes, using 15 pin power cable (the one for SATA drive) is better than 4pin to 15 pin converter (IDE to SATA converter).

Improve ZFS Performance: Step 5


ZFS supports compressing the data on the fly. This is a nice feature that improves the I/O speed – only if you have a high speed CPU (such as Quad core or higher). If your CPU is not fast enough, I don’t recommend you to turn on the compression feature, because the benefit from reducing the file size is smaller than the time spent on the CPU computation. Also, the compression algorithm plays an important role here. ZFS supports two compression algorithms, LZJB and GZIP. I personally use lz4 (See LZ4 vs LZJB for more information) because it gives a better balance between the compression radio and the performance. You can also use GZIP and specify your own compression ratio (i.e., GZIP-N). FYI, I tried GZIP-9 (The maximum compression ratio available) and I found that the overall performance gets worse even on my i7 with 12GB of memory.

There is no solid answer here because it all depends on what kind of files you store. Different files such as large file, small files, already compressed files (such as mp4 video) need different compression settings.

If you cannot decide, just go with lz4. It can’t be wrong:

#Try to use lz4 first.
sudo zfs set compression=lz4 mypool

#If you system does not support lz4, try to use lzjb
sudo zfs set compression=lzjb mypool

Improve ZFS Performance: Step 6

Identify the bottle neck

Sometimes, the limit of the ZFS I/O speed is closely related to the hardware. For example, I set up a network file system (NFS) that is based on ZFS. Most of the time, I mainly transfer the data in between the servers, rather than within the same server. Therefore, the maximum I/O speed I could get is the capacity of my network card, which is 125MB/s. On average, I can reach to 100-110 MB/s, which is pretty good for consumer grade network adapter.

One day, I decide to explore the options on network bonding, which combines multiple network adapters together on the same server. In theory, it multiplies the bandwidth, and it will increase the ZFS I/O speed.

I set up network bonding on a machine with two network cards. It was a CentOS 7 box with bonding mode: 6 (adaptive load balancing). I was able to double the I/O speed in both network and ZFS.

sudo cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.7.1 (April 27, 2011)

Bonding Mode: adaptive load balancing
Primary Slave: None
Currently Active Slave: em2
MII Status: up
MII Polling Interval (ms): 1
Up Delay (ms): 0
Down Delay (ms): 0

Slave Interface: em1
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 20:27:47:91:a3:a8
Slave queue ID: 0

Slave Interface: em2
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 20:27:47:91:a3:aa
Slave queue ID: 0

Improve ZFS Performance: Step 7

Keep your ZFS up to date

By default, ZFS will not update the file system itself even if a newer version is available on the system. For example, I created a ZFS file system on FreeBSD 8.1 with ZFS version 14. After upgrading to FreeBSD 8.2 (which supports ZFS version 15), my ZFS file system was still on version 14. I needed to upgrade it manually using the following commands:

sudo zfs upgrade my_pool
sudo zpool upgrade my_pool

Improve ZFS Performance: Step 8

Understand How the ZFS Caching Works

ZFS has three types of cache, ARC and L2ARC. ARC is a ram-based cache, and L2ARC is disk-based cache. If you want to have a super-fast ZFS system, you will need A LOT OF memory. How much? I have a RHEL 7 based data center running NFS on top of a ZFS file system. Its the storage capacity is 48TB (8 x 8TB, running RAIDZ1), and I have 96GB of memory. The I/O is about 1.5TB a day. Is the memory too much? I can tell you that sometimes it causes kernel problem because of running out of memory.

In general, you will need lots of memory for ARC cache (ram-based), and L2ARC (disk-based) is optional. Depending on which operating system you are using (in my case, I use RHEL 7 and FreeBSD), the settings can be very different. Let’s talk about the arc cache first.

By default, when you read a file from ZFS for the first time, ZFS will read the file from the disk. However if it is frequently used, ZFS will put the file in the ARC cache, which is your memory. Now we have something interesting. How much memory should you allow ZFS to use for ARC caching? You don’t want to use too little because it will increase accessing the disk, which will slow down the system. On the other hand, you don’t want to use too much because you want to reserve the memory for your operating system and other services. For FreeBSD, depend on which version you are using, it will either use 90% of the memory for ARC, or use all but 1GB of memory. For ZFS on Linux with RHEL 7, it will use about 50% of the memory.

You can monitor the ARC usage here:

zfs-stats -A

ZFS Subsystem Report                            Sun Feb 12 22:34:19 2017

ARC Summary: (HEALTHY)
        Memory Throttle Count:                  0

ARC Misc:
        Deleted:                                247.52m
        Recycle Misses:                         0
        Mutex Misses:                           68.86k
        Evict Skips:                            778.88k

ARC Size:                               75.04%  16.76   GiB
        Target Size: (Adaptive)         74.94%  16.74   GiB
        Min Size (Hard Limit):          12.50%  2.79    GiB
        Max Size (High Water):          8:1     22.33   GiB

ARC Size Breakdown:
        Recently Used Cache Size:       73.41%  12.30   GiB
        Frequently Used Cache Size:     26.59%  4.46    GiB

ARC Hash Breakdown:
        Elements Max:                           1.40m
        Elements Current:               68.50%  956.10k
        Collisions:                             50.63m
        Chain Max:                              7
        Chains:                                 94.61k


#ZFS on Linux

    time  read  miss  miss%  dmis  dm%  pmis  pm%  mmis  mm%  arcsz     c
22:33:36     0     0      0     0    0     0    0     0    0    53G   53G

If you are using FreeBSD (the most advanced operating system in the world), you don’t need to tweak your system settings at all. ZFS is part of the FreeBSD kernel, and FreeBSD has excellent memory management. Therefore, you do not need to worry about ZFS using too much memory of your system. In fact, I’ve never crashed a FreeBSD system because of using too much memory. If you know how to do it, please show me how to do it.

If you want to tweak the ARC size, you can do it via /boot/loader.conf:

#Min: 10GB

#Max: 16GB

If you are using Linux, you may want to do some extra work to make your system stable. The default settings will get you to the bottom of the mountain, and you will need to do some climbing to reach the peak.

First, you want to check the arc size:

arcstat (or depending on the version of your ZFS on Linux)

Now you will need to think about how much memory you want to reserve for ZFS. This is a tough question, and you can’t really expect Linux works like FreeBSD. Remember, Linux does not have a good memory management system like FreeBSD. It is very easy to crash the system if you do something wrong. That is one of the reasons why ZFS on Linux uses only 50% of the memory by default, because they know Linux (Linux is not a high performance operating system without some tweaking). This could be the reason why Red Hat (the maker of RHEL) does not support ZFS on Linux. Anyway, here is how to tweak the memory:

sudo nano /etc/modprobe.d/zfs.conf

#Min: 4GB
options zfs zfs_arc_min=4000000000

#Max: 8GB (Don't exceed maximum amount of ram minus 4GB)
options zfs zfs_arc_max=8000000000

Don’t forget to reboot the server.

Many people suggest using a 2GB rule, i.e., you reserve 2GB of memory for your system, and you leave the rest to ZFS. For example, if you have 32GB of memory, you may set the maximum to 30GB. Personally I don’t recommend this as it is too risky. I have multiple Linux boxes running as light NFS servers (each NFS server has 32GB of memory, four HDD with SSD disk as ZFS read cache, serves no more than 3 clients). For some odd reasons, I’ve noticed low memory warning and relatively high swap usage. Of course, this is casuing the kenel OOM killer to step in, killing my running processes until the Linux kernel can allocate memory again. Some people think this is a Linux kernel bug and they hope it will be fixed.

When I set up Linux, I highly recommend having at least 20GB for the swap. If for any reason the ZFS is eating all of your available memory, you still have swap as backup. Otherwise your system will crash.

FYI, tweaking the memory usage of ZFS ARC is more than entering some numbers. You need to think of the whole picture. Memory is a very valuable resource. You will need to decide how do use them effectively and efficiently. You can’t expect a single server running web server, database, NFS, virtual machine host all together on top of a ZFS system, and giving you a very high performance at the mean time. You need to distribute the workload across multiple servers. For example, here is what I use in a production environment:

Server 1: A NFS host
– Running NFS only
– Have network bonding based on multiple network interfaces. This will multiply the bandwidth.
– Support other servers such as web server, databases, virtual machine host etc.

Server 2: A virtual machine host
– Running Virtual Box
– Guest systems that have higher I/O requirement are hosted in a local SSD drive with standard partitions (non-ZFS).
– Guest systems that have lower I/O requirement are hosted on a different ZFS server, connected via NFS.
– All of the system memory will be used for running virtual machines.

Server 3: A web server
– Running Apache + PHP + MySQL
– The PHP code is hosted locally.
– The large static files is hosted on the NFS server. In my case, I have over 20TB static content.
– All of the system memory will be used for running Apache / PHP / MySQL executions.

Of course, I am not talking about ZFS does not work well with Apache / PHP / MySQL. I am just saying that in some extreme environments, the services should be hosted across multiple machines instead of one single machine.

Now if you have extra resources (budget and extra SATA ports), you may consider using L2ARC cache. Basically L2ARC cache is a very fast hard drive (i.e., SSD). Think of it as a hybrid hard drive, it is a buffer for reading and writing.

So what is the role of L2ARC to ARC? In general, the most frequently used files are stored in the ARC cache (ram). For the less frequently used files, or if the files are too large to fit in the arc cache, they will be stored in the L2ARC cache.

To improve the reading performance:

sudo zpool add myzpool cache 'ssd device name'

To improve the writing performance:

sudo zpool add myzpool log /dev/ssd_drive

It was impossible to remove the log devices without losing the data until ZFS v.19 (FreeBSD 8.3+/9.0+). I highly recommend to add the log drives as a mirror, i.e.,

sudo zpool add myzpool log mirror /dev/log_drive1 /dev/log_drive2

Now you may ask a question. How about using a ram disk as log / cache devices? First, ZFS already uses your system memory for I/O, so you don’t need to set up a dedicated ram disk by yourself. Also, using a ram disk for log (writing) devices is not a good idea. When somethings go wrong, such as power failure, you will end up losing your data during the writing.

Improve ZFS Performance: Step 9

More Disk = Faster

Do you know ZFS works faster on multiple devices pool than single device pool, even they have the same storage size?

In general, if you are using RAIZ1, the more disk you have, the better the overall performance. Imagine we have two different setups: One with many disks, and another one with few disks. Assuming the disk are standard consumer grade SATA HDD disk (i.e., the maximum achievable speed is around 100MB/s). Let’s say we need to calculate a checksum of a large file (e.g., 10GB). Here are the steps:

  • CPU tells the ZFS to get the file
  • The ZFS gets the file from each disk.
  • Each disk is looking for the file:
    • Setup #1: It has many disks. Each disk contains a small part of the file. It takes shorter time to return the file to the CPU.
    • Setup #2: It has fewer disks. Each disk contains a larger part of the file. It takes longer time to return the file to the CPU.
  • The CPU joins the pieces together and calculates the checksum.

As you can tell, the bottleneck of such simple operation is not the CPU, but retrieving the file. The maximum speed of standard HDD is around 100MB/s. So the more disks you have, the faster the I/O. This is the reason why we need to rebalance the ZFS every once a while, because we want to keep the data distributed evenly across all disks.

Here is a quick comparison. I had two servers available. Both had exact the same software installed (same OS/ZFS/DKMS/kernel). One had a slower CPU and one had a faster CPU. The main difference was the ZFS setup. I performed MD5 checksum on 10 files (these files are available on both systems. The total size is 70GB). They were not in the memory cache before running the checksum. Here is the result:

Setup #1: Slower CPU (4th generation i7) + ZFS with more disks.
CPU: Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz
Memory: 32GB
OS: CentOS 7
ZFS Setup: 16 HDDs: RAIDZ1 (8 HDDs) + RAIDZ1 (8 HDDs), no cache/log.

time md5sum *
real    2m47.139s
user    1m58.631s
sys     0m31.239s
Setup #2: Faster CPU  (8th generation i7) + ZFS with fewer disks.
CPU: Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz
Memory: 32GB
OS: CentOS 7
ZFS Setup: 3 HDDs: RAIDZ1 (3 HDDs), no cache/log.

time md5sum *
real    6m40.768s
user    3m18.477s
sys     0m33.790s

As you can tell, a good ZFS design makes a big difference. The first setup leaves the second one in dust (2.38x faster) even it has a slower CPU (4th generation i7 vs. 8th generation i7).

Improve ZFS Performance: Step 10

Use a combination of Striped and RAIDZ if speed is your first concern.

Striped design also gives the best performance. Since it offers no data protection at all, you may want to use RAIDZ (RAIDZ1, RAIDZ2, RAIDZ3) or mirror to handle the data protection. However, there are too many choices and each of them offer different degree of performance and protection level. If you want a quick answer, try to use a combination of striped and RAIDZ. I posted a very detail of comparison among Mirror, RAIDZ, RAIDZ2, RAIDZ3 and Striped here.

Here is an example of striped with RAIDZ:

zpool create -f myzpool raidz hd1 hd2 hd3 hd4 hd5 \
                        raidz hd6 hd7 hd8 hd9 hd10

#zpool status -v
        NAME        STATE     READ WRITE CKSUM
        storage     ONLINE       0     0     0
          raidz1-0  ONLINE       0     0     0
            hd1     ONLINE       0     0     0
            hd2     ONLINE       0     0     0
            hd3     ONLINE       0     0     0
            hd4     ONLINE       0     0     0
            hd5     ONLINE       0     0     0
          raidz1-1  ONLINE       0     0     0
            hd6     ONLINE       0     0     0
            hd7     ONLINE       0     0     0
            hd8     ONLINE       0     0     0
            hd9     ONLINE       0     0     0
            hd10    ONLINE       0     0     0

Improve ZFS Performance: Step 11

Distribute your free space evenly (How to rebalance ZFS)

One of the important tricks to improve ZFS performance is to keep the free space evenly distributed across all devices.

Technically, if the structure of a zpool has not been modified or alternated, you should not need to worry about the data distribution because ZFS takes care of that for you automatically. However, when you add new devices to an existing zpool, that will be a different story.

The following example was retrieved from one of my newly created system. I first created a pool and filled with data first (first group with 93% full). Later I added another vdev (second group). As you can see, the first group of disks is 93% full, and the second group of disks are only 0.2% full. Keep in mind that this number shows the capacity at the vdev level. In the other words, these numbers may not add up to 100. A perfectly distributed pool may have 80%/80% in two vdevs.

zpool list -v
#This is a wide table. You may need to scroll to the right.

NAME                                     SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
storage                                  138T  95.2T  43.1T          -         -     0%    68%  1.00x    ONLINE  -
  raidz1                                 102T  95.1T  6.77T          -         -     1%  93.3%      -  ONLINE
  raidz1                                36.4T  78.0G  36.3T          -         -     0%  0.20%      -  ONLINE

So what is the problem here? When ZFS retrieves the old data (data that was stored before adding the second vdev), the data is retrieved from the first vdev. When ZFS adding new data, the data will be stored in the second vdev (because the first vdev is almost full). ZFS won’t be able to fix this problem until some old data is deleted from the first vdev to free up the space. Otherwise ZFS will keep using the second vdev. For performance reason, we want ZFS to store the data across all vdevs instead of just one or two vdev. We want all harddrives to work together rather than some harddrives are working hard while the rest of the harddrives are doing nothing. In the other words, the more devices ZFS writes, the better the performance.

There are two ways to rebalance your ZFS data distribution.

1. Back up your data
2. Destroy the pool
3. Rebuild the pool
4. Put your data back


1. Copy your folder
2. Delete your original folder
3. Rename your folder back to the original name

The first method is the best because ZFS will balance the data completely. Depending on how much data do you have, it may take days to weeks to copy your data from one server to another server over a gigabit network (8.64TB per day, assuming the average network transfer speed is 100MB/s). You don’t want to use scp to do it because you will need to re-do everything again if the process is dropped. In my case, I use rsync:

(One single line)

#Run this command on the production server:
rsync -avzr --delete-before backup_server:/path_to_zpool_in_backup_server/ /path_to_zpool_in_production_server/

Of course, netcat is a faster way if you don’t care about the security. (scp / rsync will encrypt the data during transfer).
See here for further information

The second method offers great flexibility because it doesn’t require another server. It can be performed at any time as long as you have enough free space available. However it will take longer time and the result won’t be as good as the first one. Here is an example:

rsync -avr my_folder/ my_folder_copied/
rm -Rf my_folder/
mv my_folder_copied my_folder

Let’s me give you an example. Suppose we have two vdevs. Both of them are 100TB. We started with the vdev#1 first, fill it with data and we added vdev#2 later.

Event vdev#1 vdev#2
Original (TB)
Free Space (TB) 10 90
Used Space (TB) 90 10
After copying 10TB data 5 5
Free Space (TB) 5 85
Used Space (TB) 95 15
After deleting the original 10TB data 10 0
Free Space (TB) 15 85
Used Space (TB) 85 15

As you can see, the capacity of used space changes from 90%/10% to 85%/15% after these operations. It will take several trials to make these two numbers the same.

Keep in mind that when you clone the data, you need to make sure that the vdev1 (the old vdev) has enough free space to store the data (ZFS always splits the data evenly and stores them in all vdevs). Otherwise the cloned data will be written in vdev2, which is not what we want.

FYI, this is what I have after copying and deleting multiple large folders, one at a time. The capacities of both vdev had improved from from 93%/0% to 72%/60%.

#Group 1 capacity was 93%
#Group 2 capacity was 0%
NAME                                     SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
storage                                  138T  95.2T  43.1T          -         -     0%    68%  1.00x    ONLINE  -
  raidz1                                 102T  95.1T  6.77T          -         -     1%  93.3%      -  ONLINE
  raidz1                                36.4T  78.0G  36.3T          -         -     0%  0.20%      -  ONLINE
#After copying and deleting all larger folders one at a time
#Group 1 capacity becomes 72%
#Group 2 capacity becomes 60%
NAME                                     SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
storage                                  138T  94.8T  43.5T        -         -     0%    68%  1.00x    ONLINE  -
  raidz1                                 102T  73.1T  28.8T        -         -     0%  71.8%      -  ONLINE
  raidz1                                36.4T  21.7T  14.7T        -         -     0%  59.6%      -  ONLINE

Improve ZFS Performance: Step 12

Make your pool expandable

Setting up a ZFS system is more than a one-time job. Unless you take a very good care of your storage like how supermodels monitor their body weights, otherwise you will end up using all of the available space one day. Therefore, it is a good idea to come up a good design that it can grow in the future.

Suppose we want to build a server with maximum storage capacity, how will we start? Typically we try to put as many hard drives on a single machine as possible, i.e., it will be around 12 to 14 hard drives, which is what a typical consumer grade full tower computer case can hold. Let’s say we have 12 disks, here are couple setups which maximize storage capacity with a decent level of data safety:

Design #1: RAIDZ2

In this design, we create a giant pool and let the ZFS to take care of the rest. This pool will offer n-2 storage capacity which will allow up to 2 hard drives fail without losing any data.

Design #2: RAIDZ1 + RAIDZ1

In the second design, it offers the same level of storage capacity and a similar level of data protection. It allows up to one failure disk in each vdev. Keep in mind that the first design offers a great data protection. However, the second design will offer a better performance and greater flexibility in terms of future upgrade. Check out this article if you want to learn more about the difference in ZFS design.

First, let’s talk about the good and bad of the first design. It offers a great data security because it allows ANY two disks in the zpool to fail. However, it has couple disadvantages. ZFS works great when the number of disk of vdev is small. Ideally, the number should be smaller than 8 (Personally, I will stick with 5). In the first design, we put 12 disks in one single vdev, which will be problematic when the storage is getting full (>90%). Also, when we talk about upgrading the entire zpool, we will need to upgrade each disk one by one first. We won’t be able to use the extra space until we replace all 12 disks. This may be an issue for those who do not have budget to get 12 new disks at a time.

For the second design, it does not have the problem mentioned in the first design. The number of disk in each vdev is small (6 disks in each vdev). For those who don’t have plenty of budgets, it is okay to get six disks at a time to expand the pool.

Here is how to create the second design:

sudo zpool create myzpoolname raidz /dev/ada1 /dev/ada2 ... /dev/ada6 raidz /dev/ada7 /dev/ada8 /dev/ada9 ... /dev/ada12

Here is how to expand the pool by replacing the hard drive one by one without losing any data:

1. Shutdown the computer, replace the hard drive and turn on the computer.

2. Tell ZFS to replace the hard drive. This will force it to fill in the new hard drive with the existing data based on the check sum.
zpool replace mypool /dev/ada1

3. Resilver the pool
zpool scrub mypool 

4 Shutdown the server and replace the second ard drive again. Repeat the steps until everything is done.

5. zpool set autoexpand=on mypool 

6. Resilve the pool if needed.
zpool scrub mypool 

Improve ZFS Performance: Step 13

Backup your data on a different machine, not on the same pool

ZFS comes with a very cool feature. It allows you to save multiple copies of the same data in the same pool. This adds an additional layer on data security. However, I don’t recommend using this feature for backup purpose because it adds more work when writing the data to the disks. Also, I don’t think this is a good way to secure the data. I prefer to set up a mirror on a different server (Master-Slave). Since the chance of two machines fail at the same time is much smaller than one machine fails. Therefore, the data is safer in this settings.

Here is how I synchronize two machines together:

(Check out this guide on how to use rsyncd)

Create a script in the slave machine:
(One single line)

rsync -avzr -e ssh --delete-before master:/path/to/zpool/in/master/machine /path/to/zpool/in/slave/machine

And put this file in a cronjob, i.e.,

@daily root /path/to/

Now, you may ask a question. Should I go with strip-only ZFS (i.e., stripping only. No mirror, RAIDZ, RAIDZ2) when I set up my pool? Yes or no. ZFS allows you to mix any size of hard drive in one single pool. Unlike RAID[0,1,5,10] and concatenation, it can be any size and there is no lost in the disk space, i.e., you can connect 1TB, 2TB, 3TB into one single pool while enjoying the data-stripping (Total usable space = 6TB). It is fast (because there is no overhead such as parity etc) and simple. The only down side is that the entire pool will stop working if at least one device fails.

Let’s come back to the question, should we employ simple stripping in production environment? I prefer not. Strip-only ZFS divides all data into all vdev. If each vdev is simply a hard drive, and if one fails, there is NO WAY to get the original data back. If something screws up in the master machine, the only way is to destroy and rebuild the pool, and restore the data from the backup. (This process can takes hours to days if you have large amount of data, say 6TB.) Therefore, I strongly recommend to use at least RAIDZ in the production environment. If one device fails, the pool will keep working and no data is lost. Simply replace the bad hard drive with a good one and everything is good to go.

To minimize the downtime when something goes wrong, go with at least RAIDZ in a production environment (ideally, RAIDZ or strip-mirror).

For the backup machine, I think using simple stripping is completely fine.

Here is how to build a pool with simple stripping, i.e., no parity, mirror or anything

zpool create mypool /dev/dev1 /dev/dev2 /dev/dev3

And here is how to monitor the health

zpool status

Some websites suggest to use the following command instead:

zpool status -x

Don’t believe it! This command will return “all pools are healthy” even if one device is failed in a RAIDZ pool. In the other words, your data is healthy doesn’t mean all devices in your pool are healthy. So go with “zpool status” at any time.

FYI, it can easily takes few days to copy 10TB of data from one machine to another through a gigabit network. In case you need to restore large amount of data through the network, use rsync, not scp. I found that scp sometimes fail in the middle of transfer. Using rsync allows me to resume it at any time.

Improve ZFS Performance: Step 14

rsync or ZFS send?

So what’s the main difference between rsync and ZFS send? What’s the advantage of one over the other?

Rsync is a file level synchronization tool. It simply goes through the source, find out which files have been changed, and copy the corresponding files to the destination. Also rsync is portable and cross-platform. Unlike ZFS, rsync is available in most Unix platforms. If your backup platform does not support ZFS, you may want to go with rsync.

ZFS send is doing something similar. First, it takes a snapshot on the ZFS pool first:


zfs snapshot mypool/[email protected]

After that, you can generate a file that contains the pool and data information, copy to the new server to restore it:

#Method 1: Generate a file first
zfs send mypool/[email protected] > myZFSfile
scp myZFSfile backupServer:~/
zfs receive mypool/[email protected] < ~/myZFSfile

Or you can do everything in one single command line:

#Method 2: Do everything over the pipe (One command)
zfs send mypool/[email protected] | ssh backupServer zfs receive mypool/[email protected]

In general, the preparation time of ZFS send is much shorter than rsync, because ZFS already knows which files have been modified. Unlike rsync, a file-level tool, ZFS send does not need to go through the entire pool and find out such information. In terms of the transfer speed, both of them are similar.

So why do I prefer rsync over ZFS send (both methods)? It’s because the latter one is not practical! In method #1, the obvious issue is the storage space. Since it requires generating a file that contains your entire pool information. For example, suppose your pool is 10TB, and you have 8TB of data (i.e., 2TB of free space), if you go with method #1, you will need another 8TB of free space to store the file. In the other words, you will need to make sure that at least 50% of free space is available all the time. This is a quite expensive way to run ZFS.

What about method #2? Yes, it does not have the storage problem because it copies everything over the pipe line. However, what if the process is interrupted? It is a common thing due to high traffic in the network, high I/O to the disk etc. Worst worst case, you will need to re-do everything again, say, copying 8TB over the network, again.

rsync does not have these two problems. In rsync, it uses relatively small space for temporary storage, and in case the rsync process is interrupted, you can easily resume the process without copying everything again.

Improve ZFS Performance: Step 15

Disable dedup if you don’t have enough memory (5GB memory per 1TB storage)

Deduplication (dedup) is a space-saving technology. It works at the block level (a file can have many blocks). To explain it in simple English, if you have multiple copies of the same file in different places, it will store only one copy instead of multiple copies. Notice that dedup is not the same as compression. Check out this article: ZFS: Compression VS Deduplication(Dedup) in Simple English if you wan to learn more.

The idea of dedup is very simple. ZFS maintains an index of your files. Before writing any incoming files to the pool, it checks whether the storage has a copy of this file or not. If the file already exists, it will skip the file. With dedup enabled, instead of store 10 identical files, it stores one only copy. Unfortunately, the drawback is that it needs to check every incoming file before making any decision.

After upgrading my ZFS pool to version 28, I enabled dedup for testing. I found that it really caused huge performance hit. The writing speed over the network dropped from 80MB/s to 5MB/s!!! After disabling this feature, the speed goes up again.

sudo zfs set dedup=off your-zpool

In general, dedup is an expensive feature that requires a lot of hardware resources. You will need 5GB memory per 1TB of storage (Source). For example, if zpool is 10TB, I will need 50GB of memory! (Which I only have 12GB). Therefore, think twice before enabling dedup!

Notice that it won’t solve all the performance problem by disabling the dedup. For example, if you enable dedup before and disable it afterward, all files stored during this period are dedup dependent, even dedup is disabled. When you need to update these files (e.g., delete), the system still needs to check again the dedup index before any processing your file. Therefore, the performance issue still exists when working with these affected files. For the new files, it should be okay. Unfortunately, there is no way to find out the affected dedup files. The only way is to destroy and re-build the ZFS pool, which will clear the list of dedup files.

Improve ZFS Performance: Step 16

Reinstall Your Old System

Sometimes, reinstalling your old system from scratch may help to improve the performance. Recently, I decided to reinstall my FreeBSD box. It was an old FreeBSD box that was started with FreeBSD 6 (released in 2005, about 8 years ago from today). Although I upgraded the system every release, it already accumulated many junk and unused files. So I decide to reinstall the system from scratch. After the installation, I can tell that the system is more responsive and stable.

Before you wipe out the system, you can export the ZFS tank using the following command:

sudo zpool export mypool

After the work is done, you can import the data back:

sudo zpool import mypool

Improve ZFS Performance: Step 17

Connect your disks via high speed interface

Recently, I found that my overall ZFS system is slow no matter what I have done. After some investigations, I noticed that the bottle neck was my RAID card. Here are my suggestions:

1. Connect your disks to the ports with highest speed. For example, my PCI-e RAID card deliveries higher speed than my PCI RAID card. One way to verify the speed is by using dmesg, e.g.,

dmesg | grep MB

#Connected via PCI card. Speed is 1.5Gb/s
ad4: 953869MB  at ata2-master UDMA100 SATA 1.5Gb/s

#Connected via PCI-e card. Speed is 3.0 Gb/s
ad12: 953869MB  at ata6-master UDMA100 SATA 3Gb/s

In this case, the overall speed limit is based on the slowest one (1.5Gb/s), even the rest of my disks are 3Gb/s.

2. Some RAID cards come with some advanced features such as RAID, linear RAID, compression etc. Make sure that you disable these features first. You want to minimize the workload of the card and maximize the I/O speed. It will only slow down the overall process if you enable these additional features. You can disable the settings in the BIOS of the card. FYI, most of the RAID cards in $100 ranges are “software RAID”, i.e., they are using the system CPU to do the work. Personally, I think these fancy features are designed for Windows users. You really don’t need any of these features in Unix world.

3. Personally, I recommend any brand except Highpoint Rocketraid because of the driver issues. Some of the Highpoint Rocketraid products are not supported by FreeBSD natively. You will need to download the driver from their website first. Their driver is version-specified, e.g., they have two different set of drivers for FreeBSD 7 and 8, and both of them are not compatible with each other. One day if they decide to stop supporting the device, then you either need to stick with the old FreeBSD, or buy a new card. My conclusion: Stay away from Highpoint Rocketraid.

Improve ZFS Performance: Step 18

Do not use up all spaces

Depending on the settings / history of your zpool, you may want to maintain the free space at a certain level to avoid speed-drop issues.

Recently, I found that my ZFS system is very slow in terms of reading and writing. The speed dropped from 60MB/s to 5MB/s over the network. After some investigations, I found that the available space was around 300GB (out of 10TB), which is 3% left. Someone suggest that the safe threshold is about 10%, i.e., the performance won’t be impacted if you have at least 10% of the free space. I would say 5% is the bottom line, because I haven’t noticed any performance issues until it hits 3%.

After I free up some spaces, the speed comes back again.

I think it doesn’t make any sense not to use all of my space. So I decide to find out what caused this problem. The answer is the zpool structure.

In my old setup, I put set up a single RAIDZ vdev with 8 disks. This gives me basic data security (up to one disk fails), and maximum disk spaces (Usable space is 7 disks). However, I notice that the speed drops a lot when the available free space was 5%.

In my experiment setup, I decide to do the same thing with RAIZ2, i.e., it allows up to two disks fail, and the usable space is down to 6 disks. After filling up the pool, I found that it does not have the speed-drop problem. The I/O speed is still fast even the free space is 10GB (That’s 0.09%).

My conclusion: RAIDZ is okay up to 6 devices. If you want to add more devices, either use RAIDZ2 or split them into multiple vdevs:

#Suppose I have 8 disks (/dev/hd1 ... /dev/hd8).

#One vdev
zpool create myzpool raidz2 /dev/hd1 /dev/hd2 ... /dev/hd8

#Two vdevs
zpool create myzpool raidz /dev/hd1 ... /dev/hd4 raidz /dev/hd5 ... /dev/hd8

Improve ZFS Performance: Step 19


Typically, there is a setting to control how the motherboard interacts with the hard drives: IDE or AHCI. If your motherboard has IDE ports (or manufactured before 2009), it is likely that the default value is set to IDE. Try to change to AHCI. Believe me, this litter tweak can save you countless of hours on debugging.

FYI, here is my whole story.

Improve ZFS Performance: Step 20

Refresh your pool

I had set up my zpool for five years. Over the past five years, I had performed lots of upgrade and changed a lot of settings. For example, during the initial set up, I didn’t enable the compression. Later, I set the compression to lzjb and changed it to lz4. I also enabled and disabled the dedup. So you can imagine some part of the data is compressed using lzjb, some data has dedup enabled. In short, the data in my zpool has all kind of different settings. That’s dirty.

The only thing I can clean up is to destroy the entire zpool and rebuild the whole thing. Depending on the size of your data, it can take 2-3 days to transfer 10TB of data from one to another server, i.e., 4-6 days round trip. However, you will see the performance gain in long run.

Keep in mind that this step is completely optional, and it all depends on your current ZFS pool status. I have a ZFS server running on professional hardware for over 10 years, high I/O traffic every day, with over 50TB of data, and it is still running strong.

Improve ZFS Performance: Step 21

Great performance settings

The following settings will greatly improve the performance of your ZFS pool. However, each of them comes with a price tag. Its like removing the air bag from your car. Yes, it will save few pounds here and few pounds there. However, when something goes wrong, it could be a nightmare. Do it at your own risk.

Disable the sync option

sudo zfs set sync=disabled mypool

From the man page:

“File system transactions are only committed to stable storage periodically. This option will give the highest performance. However, it is very dangerous as ZFS would be ignoring the synchronous transaction demands of applications such as databases or NFS. Administrators should only use this option when the risks are understood.”

Keep in mind that disabling the sysnc options will force the synchronous writes to be treated as asynchronous writes, i.e., this will eliminating any double write penality (e.g., ZIL). In the other words, if you have ZIL (ZFS logs device), you are virtually by passing it.

Disable the checksum

sudo zfs set checksum=off mypool

Disabling the checksum is like removing air bag from your car. Most people don’t do it but some people do for their own reason (e.g., Formula 1). It will save some CPU computation time, but won’t be a lot for each operation. However if you have a lot of data to write to the pool, saving a little bit in every write operation will end up a lot. There is only a catch: your data is not important and you don’t care the data integrity. For example, I do a lot of genomic analyses. Each analysis is lengthy (may last several days) and it generate lots of temporary files (e.g., output logs). Since those files are not important information in my case, I store them in a compressed, checksum disabled zpool partition to speed up the write process. Long story short. If you care your data, enable the checksum. If you don’t, disable it.

Disable the access time

sudo zfs set atime=off mypool

From the man page:

“Turning this property off avoids producing write traffic when reading files and can result in significant performance gains, though it might confuse mailers and other similar utilities.”

Customize the record size

The default record size of ZFS is 128k, meaning that ZFS will write 128k of data to a logical block. Imagine if there is only few changes in a block, what ZFS will do is to read the whole chunk, make the changes, and write the changes back to the disk (plus other overhead such as compression, checksum etc). Most database program prefers smaller size. This magic number is different for different types of engine.

#For MySQL MyISAM Engines
sudo zfs create -o recordsize=8k mypool/mysql_myisam

#For MySQL InnoDB
sudo zfs create -o recordsize=16k mypool/mysql_innodb

To keep things simple, I create symplic links within the MySQL folder and link to the right place, e.g.,

#ls -al /var/lib/mysql
lrwxrwxrwx   1 root  root    36 Oct 17  2019 database1-> /mypool/mysql_myisam/database1/
lrwxrwxrwx   1 root  root    36 Oct 17  2019 database2-> /mypool/mysql_innodb/database2/

Since ZFS caches the data in RAM, it may be wise to increase the amount of reserved memory for ZFS, and reduce the memory used by the database program. This will avoid the double caching, which is a waste. Plus ZFS does a way better job in terms of using memory, e.g., it compresses the data using lz4 on the fly, which not all databases do it.

Similarly, you can do the same thing for virtual machine images or other applications that write a lot of small changes to large files:

sudo zfs create -o recordsize=16k mypool/vm

Redundant Metadata

Set the redundant_metadata to most will improve the performance of random writes. The default value of redundant_metadata is all.

sudo zfs set redundant_metadata=most mypool

Enable the system attribute based xattrs

Storing xattrs as system attributes significantly decreases the amount of disk I/O. (Not available on FreeBSD)

sudo zfs set xattr=sa mypool

Improve ZFS Performance: Step 22

My Settings – Simple and Clean

I have set up over 50 servers based on ZFS. They all have different purposes. In general, they all share the same ZFS settings, and they can reach the hardware limit, e.g., ARC cache uses about 90% of the system memory, the network transfer speed maxes out the limit of the network card etc.

sudo zpool history

#I like to use raidz, and each raidz vdev contains no more than 5 disks.
zpool create -f storage raidz /dev/hd1 /dev/hd2 ... raidz /dev/hd6 /dev/hd7 ... raidz /dev/hd11 /dev/hd12

#A partition for general purposes
zfs create storage/data

#A partition for general web
zfs create storage/web

#A partition for MySQL / MYISAM tables
zfs create -o recordsize=8k storage/mysql (MYISAM tables)

#Some common settings
zfs set compression=lz4 storage
zfs set atime=off storage
zfs set redundant_metadata=most storage

#For Linux server
zfs set xattr=sa storage

A simple nload when running rsync over a gigabit LAN:

(Bonding mode: 6, based on two gigabit network cards)
#nload -u M

Curr: 209.48 MByte/s
Avg: 207.50 MByte/s
Min: 198.66 MByte/s
Max: 209.53 MByte/s
Ttl: 3755.46 GByte

I only uses two operating systems:

FreeBSD 10.3-RELEASE-p11 FreeBSD 10.3-RELEASE-p11 #0: Mon Oct 24 18:49:24 UTC 2016     [email protected]:/usr/obj/usr/src/sys/GENERIC  amd64

CentOS Linux release 7.3.1611 (Core)
Linux 3.10.0-514.6.1.el7.x86_64 #1 SMP Wed Jan 18 13:06:36 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

Most of the hard drives I use are consumer-grade. Typically, I buy the external one and remove the hard drive from the enclosure because of the lower cost. Sometimes when I need to use all of the SATA ports, I will use USB flash drive for the operating system (I have some older machines with USB 2.0 port only). As long as all of the frequently used files are in the ZFS pool, the performance is not a problem. Another thing you can is to install card size SSD drive on the mSATA port, and connect a regular hard drive to the eSATA port on the back of the motherboard using a long eSATA-SATA cable. That will help to maximize the number of hard drives. Most computer cases will fit 10-12 3.5″ hard drives, some of them (e.g., Rosewill RSV-R4000) can hold 15 3.5″ hard drives for under USD 100. Personally I was able to hold 18 3.5″ hard drives using Rosewill RSV-R4000 with some modifications, plus 3 2.5″ hard drives. The most powerful one will fit 18 hard drives with a price tag of USD 200.

The amount of the memory is going to be tricky, as it depends on what applications you want to run on your server and how much memory will be consumed by the service. It also depends on how large is your ZFS pool capacity. Here are some of the servers I have set up:

#A light weight web (Apache+MySQL+PHP), and file server (Samba)
CPU: Intel(R) Xeon(R) CPU E5-2440 v2 @ 1.90GHz (A mid-level server grade CPU from 2014)
Memory: 4GB
ZFS Pool Capacity: 1.6TB
OS: CentOS 7
Kernel: 3.10.0-514.6.1.el7.x86_64 #1 SMP Wed Jan 18 13:06:36 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

#A server for nightly backup
CPU: Intel(R) Core(TM) i7 CPU K 875  @ 2.93GHz (A gaming-grade CPU from 2010)
Memory: 8GB
ZFS Pool Capacity: 43TB
OS: CentOS 7
Kernel: 3.10.0-514.6.1.el7.x86_64 #1 SMP Wed Jan 18 13:06:36 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

#A server for analyzing genomic data, with a very high CPU usage
CPU: Intel(R) Core(TM) i7-6700 CPU @ 3.40GHz (A gaming-grade CPU from 2015)
Memory: 64GB
ZFS Pool Capacity: 8.5TB
OS: CentOS 7
Kernel: 3.10.0-514.6.1.el7.x86_64 #1 SMP Wed Jan 18 13:06:36 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

#A heavy weight network file server
CPU: Intel(R) Xeon(R) CPU E5-2430 0 @ 2.20GHz (A mid-level server grade CPU from 2012)
Memory: 96GB
ZFS Pool Capacity: 53TB
OS: CentOS 7
Kernel: 3.10.0-514.6.1.el7.x86_64 #1 SMP Wed Jan 18 13:06:36 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

#A heavy weight web server
CPU: Intel(R) Xeon(R) CPU E3-1225 v3 @ 3.20GHz (An entry-level server grade CPU from 2013)
Memory: 16GB
ZFS Pool Capacity: 1.8TB
OS: CentOS 7
Kernel: 3.10.0-514.6.1.el7.x86_64 #1 SMP Wed Jan 18 13:06:36 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

#A light weight web (Apache+MySQL+PHP), and file server (Samba)
CPU: Intel(R) Core(TM) i7 CPU 920  @ 2.67GHz  (A gaming-grade CPU from 2008)
Memory: 24GB
ZFS Pool Capacity: 25TB
OS: FreeBSD 10.3
Kernel: FreeBSD 10.3-RELEASE-p11 FreeBSD 10.3-RELEASE-p11 #0: Mon Oct 24 18:49:24 UTC 2016     [email protected]:/usr/obj/usr/src/sys/GENERIC  amd64

#A low-end backup server
CPU: Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz (An entry-level CPU from 2007)
Memory: 8GB
ZFS Pool Capacity: 15TB
OS: FreeBSD 10.3
Kernel: FreeBSD 10.3-RELEASE-p11 FreeBSD 10.3-RELEASE-p11 #0: Mon Oct 24 18:49:24 UTC 2016     [email protected]:/usr/obj/usr/src/sys/GENERIC  amd64

If you are interested in implementing network-based ZFS, please check here for details.

Enjoy ZFS.


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