Debian GNU Linux initrd documentation

Turbo Fredriksson edited this page May 13, 2016 · 1 revision

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

Booting from snapshot:

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

linux   /ROOT/debian-1@/boot/vmlinuz-3.2.0-4-amd64 root=ZFS=rpool/ROOT/debian-1@some_snapshot ro boot=zfs $bootfs quiet

This will clone the snapshot rpool/ROOT/debian-1@some_snapshot into the filesystem rpool/ROOT/debian-1_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-1_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-1@) 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 is separate filesystems (for example a separate dataset for each of /usr, /var etc), the snapshot boot code will try to find the snapshot under each of these filesystems and clone (or rollback) each of these.

Example:

rpool/ROOT/debian-1@some_snapshot
rpool/ROOT/debian-1/boot@some_snapshot
rpool/ROOT/debian-1/usr@some_snapshot
rpool/ROOT/debian-1/var@some_snapshot

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

rpool/ROOT/debian-1_some_snapshot
rpool/ROOT/debian-1/boot_some_snapshot
rpool/ROOT/debian-1/usr_some_snapshot
rpool/ROOT/debian-1/var_some_snapshot

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-1 in this example) for the mount point.

Equal level filesystems

It is also possible to have all filesystems in the same 'level' as the root filesystems:

rpool/ROOT/root
rpool/ROOT/boot
rpool/ROOT/usr
rpool/ROOT/var

This, however, require the setting of the ZFS_INITRD_ADDITIONAL_DATASETS variable in the /etc/default/zfs file. They are mounted in the order of appearance in this variable, so take care not to enter a child before it's mother.

This layout does however not allow for automatic snapshot cloning and mounting.

Also, here the mountpoint property is required to know where to mount the dataset.

It does not matter (in both variants, decedent or equal filesystems if they're 'native' ZFS filesystems or legacy. The code will figure this out automatically. There is also no need for an entry in /etc/fstab for these filesystems.