DRACUT.ZFS(7) Miscellaneous Information Manual DRACUT.ZFS(7)

overview of ZFS dracut hooks

                      parse-zfs.sh → dracut-cmdline.service 
                          |                     ↓ 
                          |                     … 
                          |                     ↓ 
                          \————————→ dracut-initqueue.service 
                                                |                      zfs-import-opts.sh 
   zfs-load-module.service                      ↓                          |       | 
     |                  |                sysinit.target                    ↓       | 
     ↓                  |                       |        zfs-import-scan.service   ↓ 
zfs-import-scan.service ↓                       ↓           | zfs-import-cache.service 
     |   zfs-import-cache.service         basic.target      |     | 
     \__________________|                       |           ↓     ↓ 
                        ↓                       |     zfs-load-key.sh 
     zfs-env-bootfs.service                     |         | 
                        ↓                       ↓         ↓ 
                 zfs-import.target → dracut-pre-mount.service 
                        |          ↑            | 
                        | dracut-zfs-generator  | 
                        |  ____________________/| 
                        |/                      ↓ 
                        |                   sysroot.mount ←—————————————— dracut-zfs-generator 
                        |                       |                                        ↓   | 
                        |                       ↓            sysroot-{usr,etc,lib,&c.}.mount | 
                        |             initrd-root-fs.target ←———————————— or                 ↓ 
                        |                       |              zfs-nonroot-necessities.service 
                        |                       ↓                                 | 
                        ↓             dracut-mount.service                        | 
       zfs-snapshot-bootfs.service              |                                 | 
                        |                       ↓                                 | 
                        ↓                       …                                 | 
       zfs-rollback-bootfs.service              |                                 | 
                        |                       ↓                                 | 
                        |               sysroot-usr.mount ←———————————————————————/ 
                        |                       | 
                        |                       ↓ 
                        |                initrd-fs.target 
                        \______________________ | 
        export-zfs.sh                      initrd.target 
              |                                 | 
              ↓                                 ↓ 
   dracut-shutdown.service                      … 
                 zfs-needshutdown.sh → initrd-cleanup.service
Compare dracut.bootup(7) for the full flowchart.

Under dracut, booting with ZFS-on-/ is facilitated by a number of hooks in the 90zfs module.
Booting into a ZFS dataset requires mountpoint=/ to be set on the dataset containing the root filesystem (henceforth "the boot dataset") and at the very least either the bootfs property to be set to that dataset, or the root= kernel cmdline (or dracut drop-in) argument to specify it.
All children of the boot dataset with canmount=on with mountpoints matching /etc, /bin, /lib, /lib??, /libx32, and /usr globs are deemed essential and will be mounted as well.
zfs-mount-generator(8) is recommended for proper functioning of the system afterward (correct mount properties, remounting, &c.).

root=zfs:dataset, root=ZFS=dataset
Use dataset as the boot dataset. All pluses (‘+’) are replaced with spaces (‘ ’).
root=zfs:AUTO, root=zfs:, root=zfs, [
After import, search for the first pool with the bootfs property set, use its value as-if specified as the dataset above.
rootfstype=zfs root=dataset
Equivalent to root=zfs:dataset.
rootfstype=zfs [
Equivalent to root=zfs:AUTO.
Mount the boot dataset with -o flags; cf. Temporary Mount Point Properties in zfsprops(7). These properties will not last, since all filesystems will be re-mounted from the real root.
If specified, dracut-zfs-generator logs to the journal.
Be careful about setting neither rootfstype=zfs nor root=zfs:dataset — other automatic boot selection methods, like systemd-gpt-auto-generator and systemd-fstab-generator might take precedent.

Execute zfs snapshot boot-dataset@snapshot-name before pivoting to the real root. snapshot-name defaults to the current kernel release.
Execute zfs snapshot -Rf boot-dataset@snapshot-name before pivoting to the real root. snapshot-name defaults to the current kernel release.
Use zgenhostid(8) to set the host ID to host-id; otherwise, /etc/hostid inherited from the real root is used.
zfs_force, zfs.force, zfsforce
Appends -f to all zpool import invocations; primarily useful in conjunction with spl_hostid=, or if no host ID was inherited.

parse-zfs.sh (cmdline)
Processes spl_hostid=. If root= matches a known pattern, above, provides /dev/root and delays the initqueue until zfs(4) is loaded,
zfs-import-opts.sh (systemd environment generator)
Turns zfs_force, zfs.force, or zfsforce into ZPOOL_IMPORT_OPTS=-f for zfs-import-scan.service or zfs-import-cache.service.
zfs-load-key.sh (pre-mount)
Loads encryption keys for the boot dataset and its essential descendants.
Is prompted for via systemd-ask-password thrice.
keylocation=https://URL, keylocation=http://URL
network-online.target is started before loading.
If path doesn't exist, udevadm is settled. If it still doesn't, it's waited for for up to 10s.
zfs-env-bootfs.service (systemd service)
After pool import, sets BOOTFS= in the systemd environment to the first non-null bootfs value in iteration order.
dracut-zfs-generator (systemd generator)
Generates sysroot.mount (using rootflags=, if any). If an explicit boot dataset was specified, also generates essential mountpoints (sysroot-etc.mount, sysroot-bin.mount, &c.), otherwise generates zfs-nonroot-necessities.service which mounts them explicitly after /sysroot using BOOTFS=.
zfs-snapshot-bootfs.service, zfs-rollback-bootfs.service (systemd services)
Consume bootfs.snapshot and bootfs.rollback as described in CMDLINE. Use BOOTFS= if no explicit boot dataset was specified.
zfs-needshutdown.sh (cleanup)
If any pools were imported, signals that shutdown hooks are required.
export-zfs.sh (shutdown)
Forcibly exports all pools.
/etc/hostid, /etc/zfs/zpool.cache, /etc/zfs/vdev_id.conf (regular files)
Included verbatim, hostonly.
mount-zfs.sh (mount)
Does nothing on systemd systems (if dracut-zfs-generator succeeded). Otherwise, loads encryption key for the boot dataset from the console or via plymouth. It may not work at all!

dracut.bootup(7), zfsprops(7), zpoolprops(7), dracut-shutdown.service(8), systemd-fstab-generator(8), systemd-gpt-auto-generator(8), zfs-mount-generator(8), zgenhostid(8)
April 4, 2022 Debian