DRACUT.ZFS(7)

NAME
dracut.zfs - overview of ZFS dracut hooks

SYNOPSIS
                          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
                            |                       |
                            |                       ↓
                            |             initrd-root-fs.target ←— zfs-nonroot-necessities.service
                            |                       |                                 |
                            |                       ↓                                 |
                            ↓             dracut-mount.service                        |
           zfs-snapshot-bootfs.service              |                                 |
                            |                       ↓                                 |
                            ↓                       …                                 |
           zfs-rollback-bootfs.service              |                                 |
                            |                       ↓                                 |
                            |          /sysroot/{usr,etc,lib,&c.} ←———————————————————/
                            |                       |
                            |                       ↓
                            |                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.

DESCRIPTION
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.).

CMDLINE

  Standard
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, [root=]
    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 [root=]
    Equivalent to root=zfs:AUTO.
rootflags=flags
    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.
debug
    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.

  ZFS-specific
bootfs.snapshot[=snapshot-name]
    Execute zfs snapshot boot-dataset@snapshot-name before pivoting to the real root. snapshot-name defaults to the current kernel release.
bootfs.rollback[=snapshot-name]
    Execute zfs rollback -Rf boot-dataset@snapshot-name before pivoting to the real root. snapshot-name defaults to the current kernel release.
spl_hostid=host-id
    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.

FILES

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.
    keylocation=prompt
        Is prompted for via systemd-ask-password thrice.
    keylocation=https://URL, keylocation=http://URL
        network-online.target is started before loading.
    keylocation=file://path
        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!

SEE ALSO
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)