Fun with ZFS, part 2: Creating Debian ZFS Rescue USB Image

While ZFS is supposedly very stable and reliable it’s not an integral part of Linux and likely never will be (due to licensing issues). Because of that, upgrading kernel or the operating system may go wrong. Well, so far in my case of Debian Jessie it was very smooth, but you never know. I have heard of people whose experience was not that great.

Since I plan to do an OS upgrade, I want to be ready for the case the upgrade go wrong and I won’t be able to boot the OS stored on ZFS filesystem. In that case, one needs a a bootable live system with ZFS ready-to-use. Ideally the same version you’re already running (or planning to run) on the system one wants to upgrade.

In case of Debian, creating such an image turned out to be very very simple…

Creating Debian Live Rescue Image with ZFS

Debian have tool named live-build 1. The easies way to get is just

sudo apt-get install live-build

Once installed:

1. Create a (temporary) working directory where it all happens:

   mkdir /tmp/debian-streatch-live+zfs
   cd /tmp/debian-streatch-live+zfs
   

2. Generate initial configuration:

   lb config \
     --distribution stretch \
     --architectures=amd64 \
     --binary-images iso-hybrid \
     --iso-volume "Debian Stretch ZFS Rescue Live" \
    	   --archive-areas "main contrib" \
    	   --linux-packages "linux-image linux-headers"
   

   We should include contrib in archives since that’s where ZFS packages are. You may want to pass --backports if you want to include backports repositories. The --linux-packages is to include kernel headers necessary to compile ZFS kernel modules.

3. Configure additional packages to be installed, most importantly the ZFS packages:

   echo "
   zfs-dkms
   less
   curl
   wget
   openssh-client
   openssh-server
   openssh-sftp-server
   " >> config/package-lists/my.list.chroot
   

4. Build the live root filesystem:

   sudo lb bootstrap
   sudo lb chroot
   

5. Build the hybrid ISO binary:

   sudo lb binary
   

   After this step, you should find the resulting ISO image in live-image-amd64.hybrid.iso.

6. Profit!

Using the Rescue Image

By default, ZFS kernel module is not loaded, so:

sudo modprobe zfs

Then you need to import pools:

sudo zpool import

It may happen that pools were not cleanly exported. Then you’d have to import them with -f flag (a.k.a. force):

sudo zpoool import -f tank

You may check status of pools by:

sudo zpool status

Only do so if you’re absolutely sure no other system is accessing the pool at the time. But if you run ZFS normal PC with couple SATA disks attached like me and the PC is currently running the live rescue image than you should be fine.

Mount host’s root filesystem to /tmp/host:

mkdir /tmp/host
mount -t zfs tank/host/root /dev/host

Now you can chroot /dev/host /bin/bash and do what you want to create ZFS snapshot, whatever you wish. Just make sure that once you’re done you export pools so they can be cleanly imported by the host.

sudo zpool export tank

Some Comments

1. The configuration for lb command is stored in config directory. The lb config with various parameters just modifies (or creates if it does not exit) contents of the config. It may be a good idea to put the config into under version control such as Mercurial or git.

2. The live image produced as above is really minimal, there’s not even a man. You may want to add more packages. To do so, simply add them to the config/package-lists/my.list.chroot. You may pull in a whole GNOME by adding gnome-desktop package (though I have not tried).

To wrap it up, the process of building live Debian image is surprisingly easy. Kudos to Debian people!