contrib/dracut/README.dracut.markdown

   1 How to setup a zfs root filesystem using dracut
   2 -----------------------------------------------
   3
   4 1) Install the zfs-dracut package.  This package adds a zfs dracut module
   5 to the /usr/share/dracut/modules.d/ directory which allows dracut to
   6 create an initramfs which is zfs aware.
   7
   8 2) Set the bootfs property for the bootable dataset in the pool.  Then set
   9 the dataset mountpoint property to '/'.
  10
  11     $ zpool set bootfs=pool/dataset pool
  12     $ zfs set mountpoint=/ pool/dataset
  13
  14 Alternately, legacy mountpoints can be used by setting the 'root=' option
  15 on the kernel line of your grub.conf/menu.lst configuration file.  Then
  16 set the dataset mountpoint property to 'legacy'.
  17
  18     $ grub.conf/menu.lst: kernel ... root=ZFS=pool/dataset
  19     $ zfs set mountpoint=legacy pool/dataset
  20
  21 3) To set zfs module options put them in /etc/modprobe.d/zfs.conf file.
  22 The complete list of zfs module options is available by running the
  23 _modinfo zfs_ command.  Commonly set options include: zfs_arc_min,
  24 zfs_arc_max, zfs_prefetch_disable, and zfs_vdev_max_pending.
  25
  26 4) Finally, create your new initramfs by running dracut.
  27
  28     $ dracut --force /path/to/initramfs kernel_version
  29
  30 Kernel Command Line
  31 -------------------
  32
  33 The initramfs' behavior is influenced by the following kernel command line
  34 parameters passed in from the boot loader:
  35
  36 * `root=...`: If not set, importable pools are searched for a bootfs
  37 attribute.  If an explicitly set root is desired, you may use
  38 `root=ZFS:pool/dataset`
  39
  40 * `zfs_force=0`: If set to 1, the initramfs will run `zpool import -f` when
  41 attempting to import pools if the required pool isn't automatically imported
  42 by the zfs module.  This can save you a trip to a bootcd if hostid has
  43 changed, but is dangerous and can lead to zpool corruption, particularly in
  44 cases where storage is on a shared fabric such as iSCSI where multiple hosts
  45 can access storage devices concurrently.  _Please understand the implications
  46 of force-importing a pool before enabling this option!_
  47
  48 * `spl_hostid`: By default, the hostid used by the SPL module is read from
  49 /etc/hostid inside the initramfs.  This file is placed there from the host
  50 system when the initramfs is built which effectively ties the ramdisk to the
  51 host which builds it.  If a different hostid is desired, one may be set in
  52 this attribute and will override any file present in the ramdisk.  The
  53 format should be hex exactly as found in the `/etc/hostid` file, IE
  54 `spl_hostid=0x00bab10c`.
  55
  56 Note that changing the hostid between boots will most likely lead to an
  57 un-importable pool since the last importing hostid won't match.  In order
  58 to recover from this, you may use the `zfs_force` option or boot from a
  59 different filesystem and `zpool import -f` then `zpool export` the pool
  60 before rebooting with the new hostid.
  61
  62 How it Works
  63 ============
  64
  65 The Dracut module consists of the following files (less Makefile's):
  66
  67 * `module-setup.sh`: Script run by the initramfs builder to create the
  68 ramdisk.  Contains instructions on which files are required by the modules
  69 and z* programs.  Also triggers inclusion of `/etc/hostid` and the zpool
  70 cache.  This file is not included in the initramfs.
  71
  72 * `90-zfs.rules`: udev rules which trigger loading of the ZFS modules at boot.
  73
  74 * `zfs-lib.sh`: Utility functions used by the other files.
  75
  76 * `parse-zfs.sh`: Run early in the initramfs boot process to parse kernel
  77 command line and determine if ZFS is the active root filesystem.
  78
  79 * `mount-zfs.sh`: Run later in initramfs boot process after udev has settled
  80 to mount the root dataset.
  81
  82 * `export-zfs.sh`: Run on shutdown after dracut has restored the initramfs
  83 and pivoted to it, allowing for a clean unmount and export of the ZFS root.
  84
  85 `zfs-lib.sh`
  86 ------------
  87
  88 This file provides a few handy functions for working with ZFS. Those
  89 functions are used by the `mount-zfs.sh` and `export-zfs.sh` files.
  90 However, they could be used by any other file as well, as long as the file
  91 sources `/lib/dracut-zfs-lib.sh`.
  92
  93 `module-setup.sh`
  94 -----------------
  95
  96 This file is run by the Dracut script within the live system, not at boot
  97 time.  It's not included in the final initramfs.  Functions in this script
  98 describe which files are needed by ZFS at boot time.
  99
 100 Currently all the various z* and spl modules are included, a dependency is
 101 asserted on udev-rules, and the various zfs, zpool, etc. helpers are included.
 102 Dracut provides library functions which automatically gather the shared libs
 103 necessary to run each of these binaries, so statically built binaries are
 104 not required.
 105
 106 The zpool and zvol udev rules files are copied from where they are
 107 installed by the ZFS build.  __PACKAGERS TAKE NOTE__: If you move
 108 `/etc/udev/rules/60-z*.rules`, you'll need to update this file to match.
 109
 110 Currently this file also includes `/etc/hostid` and `/etc/zfs/zpool.cache`
 111 which means the generated ramdisk is specific to the host system which built
 112 it.  If a generic initramfs is required, it may be preferable to omit these
 113 files and specify the `spl_hostid` from the boot loader instead.
 114
 115 `parse-zfs.sh`
 116 --------------
 117
 118 Run during the cmdline phase of the initramfs boot process, this script
 119 performs some basic sanity checks on kernel command line parameters to
 120 determine if booting from ZFS is likely to be what is desired.  Dracut
 121 requires this script to adjust the `root` variable if required and to set
 122 `rootok=1` if a mountable root filesystem is available.  Unfortunately this
 123 script must run before udev is settled and kernel modules are known to be
 124 loaded, so accessing the zpool and zfs commands is unsafe.
 125
 126 If the root=ZFS... parameter is set on the command line, then it's at least
 127 certain that ZFS is what is desired, though this script is unable to
 128 determine if ZFS is in fact available.  This script will alter the `root`
 129 parameter to replace several historical forms of specifying the pool and
 130 dataset name with the canonical form of `zfs:pool/dataset`.
 131
 132 If no root= parameter is set, the best this script can do is guess that
 133 ZFS is desired.  At present, no other known filesystems will work with no
 134 root= parameter, though this might possibly interfere with using the
 135 compiled-in default root in the kernel image.  It's considered unlikely
 136 that would ever be the case when an initramfs is in use, so this script
 137 sets `root=zfs:AUTO` and hopes for the best.
 138
 139 Once the root=... (or lack thereof) parameter is parsed, a dummy symlink
 140 is created from `/dev/root` -> `/dev/null` to satisfy parts of the Dracut
 141 process which check for presence of a single root device node.
 142
 143 Finally, an initqueue/finished hook is registered which causes the initqueue
 144 phase of Dracut to wait for `/dev/zfs` to become available before attempting
 145 to mount anything.
 146
 147 `mount-zfs.sh`
 148 --------------
 149
 150 This script is run after udev has settled and all tasks in the initqueue
 151 have succeeded.  This ensures that `/dev/zfs` is available and that the
 152 various ZFS modules are successfully loaded.  As it is now safe to call
 153 zpool and friends, we can proceed to find the bootfs attribute if necessary.
 154
 155 If the root parameter was explicitly set on the command line, no parsing is
 156 necessary.  The list of imported pools is checked to see if the desired pool
 157 is already imported.  If it's not, and attempt is made to import the pool
 158 explicitly, though no force is attempted.  Finally the specified dataset
 159 is mounted on `$NEWROOT`, first using the `-o zfsutil` option to handle
 160 non-legacy mounts, then if that fails, without zfsutil to handle legacy
 161 mount points.
 162
 163 If no root parameter was specified, this script attempts to find a pool with
 164 its bootfs attribute set.  First, already-imported pools are scanned and if
 165 an appropriate pool is found, no additional pools are imported.  If no pool
 166 with bootfs is found, any additional pools in the system are imported with
 167 `zpool import -N -a`, and the scan for bootfs is tried again.  If no bootfs
 168 is found with all pools imported, all pools are re-exported, and boot fails.
 169 Assuming a bootfs is found, an attempt is made to mount it to `$NEWROOT`,
 170 first with, then without the zfsutil option as above.
 171
 172 Ordinarily pools are imported _without_ the force option which may cause
 173 boot to fail if the hostid has changed or a pool has been physically moved
 174 between servers.  The `zfs_force` kernel parameter is provided which when
 175 set to `1` causes `zpool import` to be run with the `-f` flag.  Forcing pool
 176 import can lead to serious data corruption and loss of pools, so this option
 177 should be used with extreme caution.  Note that even with this flag set, if
 178 the required zpool was auto-imported by the kernel module, no additional
 179 `zpool import` commands are run, so nothing is forced.
 180
 181 `export-zfs.sh`
 182 ---------------
 183
 184 Normally the zpool containing the root dataset cannot be exported on
 185 shutdown as it is still in use by the init process. To work around this,
 186 Dracut is able to restore the initramfs on shutdown and pivot to it.
 187 All remaining process are then running from a ramdisk, allowing for a
 188 clean unmount and export of the ZFS root. The theory of operation is
 189 described in detail in the [Dracut manual](https://www.kernel.org/pub/linux/utils/boot/dracut/dracut.html#_dracut_on_shutdown).
 190
 191 This script will try to export all remaining zpools after Dracut has
 192 pivoted to the initramfs. If an initial regular export is not successful,
 193 Dracut will call this script once more with the `final` option,
 194 in which case a forceful export is attempted.
 195
 196 Other Dracut modules include similar shutdown scripts and Dracut
 197 invokes these scripts round-robin until they succeed. In particular,
 198 the `90dm` module installs a script which tries to close and remove
 199 all device mapper targets. Thus, if there are ZVOLs containing
 200 dm-crypt volumes or if the zpool itself is backed by a dm-crypt
 201 volume, the shutdown scripts will try to untangle this.