Debian GNU Linux initrd documentation

Supported boot parameters

  • rollback=<on|yes|1> Do a rollback of specified snapshot.

  • zfs_debug=<on|yes|1> Debug the initrd script

  • zfs_force=<on|yes|1> Force importing the pool. Should not be necessary.

  • zfs=<off|no|0> Don’t try to import ANY pool, mount ANY filesystem or even load the module.

  • rpool=<pool> Use this pool for root pool.

  • bootfs=<pool>/<dataset> Use this dataset for root filesystem.

  • root=<pool>/<dataset> Use this dataset for root filesystem.

  • root=ZFS=<pool>/<dataset> Use this dataset for root filesystem.

  • root=zfs:<pool>/<dataset> Use this dataset for root filesystem.

  • root=zfs:AUTO Try to detect both pool and rootfs

In all these cases, <dataset> could also be <dataset>@<snapshot>.

The reason there are so many supported boot options to get the root filesystem, is that there are a lot of different ways too boot ZFS out there, and I wanted to make sure I supported them all.

Pool imports

Import using /dev/disk/by-*

The initrd will, if the variable USE_DISK_BY_ID is set in the file /etc/default/zfs, to import using the /dev/disk/by-* links. It will try to import in this order:

  1. /dev/disk/by-vdev

  2. /dev/disk/by-*

  3. /dev

Import using cache file

If all of these imports fail (or if USE_DISK_BY_ID is unset), it will then try to import using the cache file.

Last ditch attempt at importing

If that ALSO fails, it will try one more time, without any -d or -c options.


Booting from snapshot:

Enter the snapshot for the root= parameter like in this example:

linux   /BOOT/debian@/boot/vmlinuz-5.10.0-9-amd64 root=ZFS=rpool/ROOT/debian@some_snapshot ro

This will clone the snapshot rpool/ROOT/debian@some_snapshot into the filesystem rpool/ROOT/debian_some_snapshot and use that as root filesystem. The original filesystem and snapshot is left alone in this case.

BEWARE that it will first destroy, blindingly, the rpool/ROOT/debian_some_snapshot filesystem before trying to clone the snapshot into it again. So if you’ve booted from the same snapshot previously and done some changes in that root filesystem, they will be undone by the destruction of the filesystem.

Snapshot rollback

From version 0.6.4-1-3 it is now also possible to specify rollback=1 to do a rollback of the snapshot instead of cloning it. BEWARE that this will destroy all snapshots done after the specified snapshot!

Select snapshot dynamically

From version 0.6.4-1-3 it is now also possible to specify a NULL snapshot name (such as root=rpool/ROOT/debian@) and if so, the initrd script will discover all snapshots below that filesystem (sans the at), and output a list of snapshot for the user to choose from.

Booting from native encrypted filesystem

Although there is currently no support for native encryption in ZFS On Linux, there is a patch floating around ‘out there’ and the initrd supports loading key and unlock such encrypted filesystem.

Separated filesystems

Descended filesystems

If there are separate filesystems (for example a separate dataset for /usr), the snapshot boot code will try to find the snapshot under each filesystems and clone (or rollback) them.



These will create the following filesystems respectively (if not doing a rollback):


The initrd code will use the mountpoint option (if any) in the original (without the snapshot part) dataset to find where it should mount the dataset. Or it will use the name of the dataset below the root filesystem (rpool/ROOT/debian in this example) for the mount point.