Recently, I built a FreeBSD server with ZFS file system using six 2TB harddisks. If you like to learn what I have done or understand more about ZFS, you can read the story here: ZFS+USB: Building a Super Large Server Using USB Memory, CF Card and SD Card..
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.
Improve ZFS Performance: Step 1
A 64-bit decent CPU + Lots of memory
Traditionally, we are told to use a less powerful computer for a file/data server. That’s not true on ZFS. ZFS is more than a file system. It uses a lot of resources to improve the performance of the input/output, such as compression 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 file is smaller, it takes shorter time to write to the disk, which results a higher writing speed. Same for the reading. ZFS can cache the file for you in the memory, which results 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 (I personally use i7 + 8GB). Of course, the faster the CPU, the more the memory, the better performance. Of course, you need a 64-bit operating system.
Don’t believe? Let’s do an experiment. 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 (8 cores) + 8GB Memory + FreeBSD 8.2 64-bit #time dd if=/dev/zero of=./file.out bs=1M count=10k 10240+0 records in 10240+0 records out 10737418240 bytes transferred in 6.138918 secs (1749073364 bytes/sec) real 0m6.140s user 0m0.023s sys 0m3.658s
That’s 1.6GB/s! What is it so fast? That’s because it is a zero based file. After the compression, a compressed 10GB file may result in several 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 8.2 64-bit #time dd if=/dev/zero of=./file.out bs=1M count=10k 10240+0 records in 10240+0 records out 10737418240 bytes transferred in 23.672373 secs (453584362 bytes/sec) real 0m23.675s user 0m0.091s sys 0m22.409s
That’s 434MB/s only. See the difference?
Improve ZFS Performance: Step 2
Tweaking the Boot Loader Parameters
Many people complain about ZFS for its stability issues, such as kernel panic, reboot randomly, crash when copying large files (> 2GB) etc. It has something to do with the boot loader settings. By default, ZFS will not work smoothly without tweaking the system parameters system. Even FreeBSD 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 an equation. You can tweak your boot loader (/boot/loader.conf) using the following parameters:
#I have 8G of Ram vfs.zfs.prefetch_disable=0 #If Ram = 4GB, set the value to 512M #If Ram = 8GB, set the value to 1024M vfs.zfs.arc_min="1024M" #Ram x 0.5 - 512 MB vfs.zfs.arc_max="3584M" #Ram x 2 vm.kmem_size_max="16G" #Ram x 1.5 vm.kmem_size="12G"
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!
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 slower disk (e.g., 5900 rpm) and a faster disk(7200 rpm) in the same virtual device (vdev), the overall speed will depend on the slowest disk. Also, different harddrives may have different sector size. For example, Western Digital releases a harddrive with 4k sector, while the general harddrives use 512 byte. Mixing harddrives with different sectors can bring performance penalty too. Here is a quick way to check the model of your harddrive:
sudo dmesg | grep ad
In my cases, I have the following:
ad10: 1907729MB Hitachi HDS722020ALA330 JKAOA20N at ata5-master UDMA100 SATA 3Gb/s ad11: 1907729MB Seagate ST32000542AS CC34 at ata5-slave UDMA100 SATA 3Gb/s ad12: 1907729MB WDC WD20EARS-00MVWB0 51.0AB51 at ata6-master UDMA100 SATA 3Gb/s ad13: 1907729MB Hitachi HDS5C3020ALA632 ML6OA580 at ata6-slave UDMA100 SATA 3Gb/s ad14: 1907729MB WDC WD20EARS-00MVWB0 50.0AB50 at ata7-master UDMA100 SATA 3Gb/s ad16: 1907729MB WDC WD20EARS-00MVWB0 51.0AB51 at ata8-master UDMA100 SATA 3Gb/s
Notice that my Western Digital harddrives with 4k sectors all ends with EARS in the model number.
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.
Improve ZFS Performance: Step 4
Compression
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 calculation. Also, the compression algorithm plays an important role here. ZFS supports two compression algorithms, LZJB and GZIP. I personally use LZJB 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 on my i7 with 8GB of memory.
There is no solid answer here because it all depends on what files you store. Different files such as large file, small files, already compressed files (such as Xvid movie) need different compression settings.
If you cannot decide, just go with lzjb. It can’t be wrong:
sudo zfs set compression=lzjb mypool
Improve ZFS Performance: Step 5
Disable the unwanted features
By default, ZFS enables a lot of settings for data security, such as checksum etc. If you don’t care about the additional data security, just disable them. You can use the following command to view your ZFS settings.
sudo zfs get all
FYI, I usually disable the following:
#I don't care about the checksum. sudo zfs set checksum=off myzpool #I don't need ZFS to update the access time when reading the file sudo zfs set atime=off myzpool
For more info, go to:
man zfs
Improve ZFS Performance: Step 6
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 7
Use Cache / Log Devices
Suppose you have a very fast SSD harddrive. You can use it for logging/caching the data for your ZFS pool.
To improve the reading performance:
sudo zpool add 'zpool name' cache 'ssd device name'
To improve the writing performance:
sudo zpool add 'zpool name' log 'ssd device name'
Since it is impossible to remove the log devices without losing the data. I highly recommend to add the log drives as a mirror, i.e.,
sudo zpool add zpool_name 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 8
Two is better than one
Do you know ZFS works faster on multiple devices pool than single device pool, even they have the same storage size?
Improve ZFS Performance: Step 9
Use Mirror, not RAIDZ if speed is your first concern.
If you need performance, go with mirror, not RAIDZ. When ZFS stores the data in a mirror pool, it simply stores the whole file in each device. When it reads the file, it simply get the partial copy from each device first, and combine them at the end. Since this process can happen in parallel, it will speed up the reading process.
On the other hand, RAIDZ works a bit differently. Suppose there are N devices in your RAIDZ pool. When the data is written to a RAIDZ pool, ZFS needs to divided it into N-1 parts first, calculate the parity, and write all of them into the N devices. When reading the data, ZFS will read the N-1 devices first, make sure that the result is okay (otherwise it will read the data again from the remaining device), and combine them together. This additional work adds more work. That’s why RAIDZ is always slower than mirror.
So this is the ideal set up (given that you have enough budget):
Command to create a mirror zpool.
sudo zpool create zpool_name mirror /dev/hd1 /dev/hd2 mirror /dev/hd3 /dev/hd4 mirror /dev/hd5 /dev/hd6
Here is another model: Strip only, very fast with no data security.
Command to create a strip only zpool.
sudo zpool create zpool_name /dev/hd1 /dev/hd2 /dev/hd3 /dev/hd4 /dev/hd5 /dev/hd6
Here is the most popular model: RAIDZ, not so fast, but with okay data security.
Command to create a RAIDZ zpool.
sudo zpool create zpool_name raidz /dev/hd1 /dev/hd2 /dev/hd3 /dev/hd4 /dev/hd5 /dev/hd6
Improve ZFS Performance: Step 10
Distribute your free space evenly
One of the important tricks to improve ZFS performance is to keep the free space evenly distributed across all devices.
You can check it using the following command:
zpool iostat -v
The free space is show on the second column (available capacity)
capacity operations bandwidth
pool used avail read write read write
---------- ----- ----- ----- ----- ----- -----
storage 3.23T 1.41T 0 3 49.1K 439K
ad4 647G 281G 0 0 5.79K 49.2K
ad8 647G 281G 0 0 5.79K 49.6K
ad10 647G 281G 0 0 5.82K 49.6K
ad16 647G 281G 0 0 5.82K 49.6K
ad18 647G 281G 0 0 5.77K 49.5K
When ZFS writes a new file to replace the old file in the system, it will first write the file in the free space first, then move the file pointer from the old one to the new one. In this case, even there is a power failure during writing the data, no data will be lost because the file pointer is still pointing to the old file. That’s why ZFS does not need fsck (file system check).
In order to keep the performance at a good level, we need to make sure that the free space is available in every device in the pool. Otherwise ZFS can only write the data to some of the devices only (instead of all). In the other words, the higher number of devices ZFS write, the better the performance.
Technically, if the structure of a zpool has not been modified or alternated, you should not need to worry about the free space distribution because ZFS will take care of that for you automatically. However, when you add a new device to an existing pool, that will be a different story, e.g.,
capacity operations bandwidth
pool used avail read write read write
---------- ----- ----- ----- ----- ----- -----
storage 3.88T 2.33T 0 3 49.1K 439K
ad4 647G 281G 0 0 5.79K 49.2K
ad8 647G 281G 0 0 5.79K 49.6K
ad10 647G 281G 0 0 5.82K 49.6K
ad16 647G 281G 0 0 5.82K 49.6K
ad18 647G 281G 0 0 5.77K 49.5K
ad20 0 928G 0 0 5.77K 49.5K
In this example, I add a 1TB hard drive (ad20) to my existing pool, which gives about 928GB of free space. Let say I add a 6GB file, the free space will look something like this:
capacity operations bandwidth
pool used avail read write read write
---------- ----- ----- ----- ----- ----- -----
storage 4.48T 1.73T 0 3 49.1K 439K
ad4 648G 280G 0 0 5.79K 49.2K
ad8 648G 280G 0 0 5.79K 49.6K
ad10 648G 280G 0 0 5.82K 49.6K
ad16 648G 280G 0 0 5.82K 49.6K
ad18 648G 280G 0 0 5.77K 49.5K
ad20 1G 927G 0 0 5.77K 49.5K
In the other words, ZFS will still divide my 6GB file into six equal pieces and write each piece to each device. Eventually, ZFS will use up the free space in the older devices, and it can write the data to the new devices only (ad20), which will decrease the performance. Unfortunately, there is no way to redistribute the data / free space evenly without destroying the pool, i.e.,
1. Back up your data 2. Destroy the pool 3. Rebuild the pool 4. Put the data back
Depending on how much data do you have, it can take 2 to 3 days to copy 5TB of data from one server to another server over a gigabit network. 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
Improve ZFS Performance: Step 11
Backup the data on different machine, not 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 for 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 sync two machines together:
Create a script in the slave machine: getContentFromMaster.sh
(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.,
/etc/crontab
@daily root /path/to/getContentFromMaster.sh
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 har ddrive in one single pool. Unlike RAIZ{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 size 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) 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 12
Disable dedup if you don’t have enough memory (2GB memory per 1TB storage)
Deduplication (dedup) is a space-saving technology. It simply stores one copy of your file instead of storing multiple copies. For example, suppose you have ten identical folders with the same files. If the dedup is enabled, ZFS only stores one copy instead of multiple copies. Notice that dedup is not the same as compression. ZFS: Compression VS Deduplication(Dedup) in Simple English
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 2GB memory per 1TB. For example, if zpool is 10TB, I will need 20GB 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 re-build/empty the ZFS pool, which will clear the list of dedup files.
Enjoy ZFS.
–Derrick
