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ZFS on Linux is a poorly designed software solution to get ZFS up and running in Linux environments. Unlike FreeBSD, ZFS does not work with the Linux kernel natively. The developers of ZFS on Linux come up a creative hack. By injecting the ZFS into the kernel via DKMS, Linux kernel will understand what is ZFS. It works very well, and it really works with a single assumption: The system will never be updated or rebooted after installing ZFS on Linux. So what will happen after you update the system such as kernel or the system got rebooted? There is a good chance that your ZFS module will not be loaded. Otherwise you won’t be reading this article, right?

Okay, I am assuming that you already said thousands of f-words to those stupid developers at ZFS on Linux. So did I. You just can’t assume that a MINI works like a semi truck. MINI was created for a totally different purposes. If you want to use a MINI like a semi-truck, then you have to accept the corresponding risks.

Long story short, below is how you get your data back:

You reboot your computer after a system upgrade (either updating to a new kernel, or updating the DKMS library, or both), and you notice that the ZFS is not loaded, i.e.,

#zpool import -a
The ZFS modules are not loaded.
Try running '/sbin/modprobe zfs' as root to load them.


#/sbin/modprobe zfs
modprobe: FATAL: Module zfs not found.



#dkms status
spl, 0.7.7, 3.10.0-693.17.1.el7.x86_64, x86_64: installed (original_module exists)
spl, 0.7.7, 3.10.0-693.21.1.el7.x86_64, x86_64: installed
zfs, 0.7.7, 3.10.0-693.17.1.el7.x86_64, x86_64: installed (original_module exists) (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!)
zfs, 0.7.7, 3.10.0-693.21.1.el7.x86_64, x86_64: installed (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!)

What you need to do is to erase all the ZFS and related packages:

yum erase zfs zfs-dkms libzfs2 spl spl-dkms libzpool2 -y

Please reboot the system. This step is very important.

reboot

After that, try to install ZFS again.

yum install zfs -y

If the system complaints about mismatch dependent packages, try to remove the affected packages first and run the installation again.

After the installation, try to start the ZFS module:

/sbin/modprobe zfs

zpool import -a

If the ZFS is up and running, you may move to next step. Otherwise, I suggest you to do the following:

• Reboot
• Clear the cache of the yum repository and try to update the system again. (sudo yum clean all)
• Reboot to the latest kernel
• Erase the ZFS and related packages and try it again.

ZFS is portable. If you are unable to get it works, try to bring the ZFS disks to other servers, the new server should be able to recognize the ZFS disks. For the original server, you can connect to the ZFS disks on the new server via NFS using the original path. That will minimize the impact of changes.

Assuming that your ZFS is up and running, we will need to check the DKMS status:

#dkms status

If you see no error message, you are good to go.

spl, 0.7.8, 3.10.0-693.21.1.el7.x86_64, x86_64: installed (original_module exists)
zfs, 0.7.8, 3.10.0-693.21.1.el7.x86_64, x86_64: installed (original_module exists)

If you see an error message like the following, you may need to rebuild the modules again:

spl, 0.7.8, 3.10.0-693.21.1.el7.x86_64, x86_64: installed (original_module exists)
zfs, 0.7.8, 3.10.0-693.21.1.el7.x86_64, x86_64: installed (original_module exists) (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!)

sudo dkms remove zfs/0.7.8 --all
sudo dkms remove spl/0.7.8 --all
sudo dkms --force install spl/0.7.8
sudo dkms --force install zfs/0.7.8
sudo dkms status

Now you understand why no professional truckers use MINI for work. Good luck!

Let’s all agree with this fact: ZFS is foreign to Linux. It is not native. You can’t expect that ZFS on Linux will run smoothly as FreeBSD or Solaris. Having using ZFS on Linux since 2013 (and ZFS on FreeBSD since 2009), I’ve noticed that ZFS does not like Linux (well, at least RHEL 7). Here are some few examples:

• ZFS is not loaded at the boot time. You will need to manually start it or load it via cron. Good luck if you have other services (like Apache, MySQL, NFS, or even users’ home directories) that depend on the ZFS.
• Every single time you update the kernel, ZFS will not work after the reboot without some manual work. What if the system runs the update automatically, and one day there is a power failure which makes your server to reboot to a new kernel? Your system will not be able to mount your ZFS volume. If you integrate ZFS with other service applications such as web, database or network drive, oh well, good luck and I hope you will catch this problem fast enough before receiving thousands of emails and calls from your end-users.
• If you exclude the kernel from the updates (/etc/yum.conf), you will eventually run into trouble, because there are tons of other packages that require the latest kernel. In the other words, running the command: yum update -y will fail. You will need to run yum update –skip-broken, which means you will miss many latest packages. Here is an example:

  --> Finished Dependency Resolution
  Error: Package: hypervvssd-0-0.29.20160216git.el7.x86_64 (base)
             Requires: kernel >= 3.10.0-384.el7
             Installed: kernel-3.10.0-327.el7.x86_64 (@anaconda)
                 kernel = 3.10.0-327.el7
             Installed: kernel-3.10.0-327.22.2.el7.x86_64 (@updates)
                 kernel = 3.10.0-327.22.2.el7
  Error: Package: hypervfcopyd-0-0.29.20160216git.el7.x86_64 (base)
             Requires: kernel >= 3.10.0-384.el7
             Installed: kernel-3.10.0-327.el7.x86_64 (@anaconda)
                 kernel = 3.10.0-327.el7
             Installed: kernel-3.10.0-327.22.2.el7.x86_64 (@updates)
                 kernel = 3.10.0-327.22.2.el7
  Error: Package: hypervkvpd-0-0.29.20160216git.el7.x86_64 (base)
             Requires: kernel >= 3.10.0-384.el7
             Installed: kernel-3.10.0-327.el7.x86_64 (@anaconda)
                 kernel = 3.10.0-327.el7
             Installed: kernel-3.10.0-327.22.2.el7.x86_64 (@updates)
                 kernel = 3.10.0-327.22.2.el7
   You could try using --skip-broken to work around the problem
   You could try running: rpm -Va --nofiles --nodigest
  

• If you are running the stable Linux distributions like RHEL 7, you can load a more recent kernel like 4.x by installing the package: kernel-ml. However, don’t expect that ZFS will work with version 4:

  Loading new spl-0.6.5.9 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/0.6.5.9/build/make.log for more information.
  
  

Running ZFS on Linux is like putting a giraffe in the wild in Alaska. It is just not the right thing to do. Unfortunately, there are so many things that only available on Linux so we have to live with it. Just like FUSE (Filesystem in Userspace), many people feel hesitated to run their file systems on the userspace instead of kernel level, but hey, see how many people are happy with GlusterFS, a distributed file system that live on FUSE! Personally I just think it is not a right thing to do, especially in an enterprise environment. Running a production file system at the userspace level, seriously?

Anyway, if you are running into trouble after upgrading your Linux kernel (and you almost had a heart attack when you think your data may be lost), you have two choices:

1. Simply boot to the previous working kernel if you need to get your data back in quick. However, keep in mind that this will create two problems:
   • Since you already update the system with the new kernel and the new packages, your new packages probably will not work with the old kernel, and that may give you extra headache.
   • Unless you manually overwrite the kernel boot order (boot loader config), otherwise you may get into the same trouble in the next boot.
2. If you want a more “permanent” fix, you will need to rebuild the dkms ZFS and SPL modules. See below for the instructions. Keep in mind that you will have the same problem again when the kernel receives a new update.

You’ve tried to load the ZFS and realize that it is no longer available:

#sudo zpool import
The ZFS modules are not loaded.
Try running '/sbin/modprobe zfs' as root to load them.

#sudo /sbin/modprobe zfs
modprobe: FATAL: Module zfs not found.

You may want to check the dkms status. Write down the version number. In my case, it is 0.6.5.9

#sudo dkms status
spl, 0.6.5.9, 3.10.0-327.28.3.el7.x86_64, x86_64: installed (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!)
spl, 0.6.5.9, 3.10.0-514.2.2.el7.x86_64, x86_64: installed (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!)
zfs, 0.6.5.9, 3.10.0-327.28.3.el7.x86_64, x86_64: installed (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!) (WARNING! Diff between built and installed module!)

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:
3.10.0-514.2.2.el7.x86_64

#Now, let's get into the fun part. We will remove them and reinstall them.
#Don't forget to match your version, in my base, my version is: 0.6.5.9
sudo dkms remove zfs/0.6.5.9 --all
sudo dkms remove spl/0.6.5.9 --all
sudo dkms --force install spl/0.6.5.9
sudo dkms --force install zfs/0.6.5.9

#or you can run these commands in one line, so that you don't need to wait:
sudo dkms remove zfs/0.6.5.9 --all; sudo dkms remove spl/0.6.5.9 --all; sudo dkms --force install spl/0.6.5.9; sudo dkms --force install zfs/0.6.5.9;

And we will verify the result.

#sudo dkms status
spl, 0.6.5.9, 3.10.0-514.2.2.el7.x86_64, x86_64: installed
zfs, 0.6.5.9, 3.10.0-514.2.2.el7.x86_64, x86_64: installed

Finally we can start the ZFS again.

sudo /sbin/modprobe zfs

Your ZFS pool should back. You can verify it by rebooting your machine. Notice that Linux may not automatically mount the ZFS volumes. You may want to mount it manually or via cron job.

Here is how to mount the ZFS volumes manually.

sudo zpool import -a

You may want to remove all of the old kernels too.

sudo package-cleanup --oldkernels --count=1 -y