201 lines
9.6 KiB
Markdown
201 lines
9.6 KiB
Markdown
How to setup a zfs root filesystem using dracut
|
|
-----------------------------------------------
|
|
|
|
1) Install the zfs-dracut package. This package adds a zfs dracut module
|
|
to the /usr/share/dracut/modules.d/ directory which allows dracut to
|
|
create an initramfs which is zfs aware.
|
|
|
|
2) Set the bootfs property for the bootable dataset in the pool. Then set
|
|
the dataset mountpoint property to '/'.
|
|
|
|
$ zpool set bootfs=pool/dataset pool
|
|
$ zfs set mountpoint=/ pool/dataset
|
|
|
|
Alternately, legacy mountpoints can be used by setting the 'root=' option
|
|
on the kernel line of your grub.conf/menu.lst configuration file. Then
|
|
set the dataset mountpoint property to 'legacy'.
|
|
|
|
$ grub.conf/menu.lst: kernel ... root=ZFS=pool/dataset
|
|
$ zfs set mountpoint=legacy pool/dataset
|
|
|
|
3) To set zfs module options put them in /etc/modprobe.d/zfs.conf file.
|
|
The complete list of zfs module options is available by running the
|
|
_modinfo zfs_ command. Commonly set options include: zfs_arc_min,
|
|
zfs_arc_max, zfs_prefetch_disable, and zfs_vdev_max_pending.
|
|
|
|
4) Finally, create your new initramfs by running dracut.
|
|
|
|
$ dracut --force /path/to/initramfs kernel_version
|
|
|
|
Kernel Command Line
|
|
-------------------
|
|
|
|
The initramfs' behavior is influenced by the following kernel command line
|
|
parameters passed in from the boot loader:
|
|
|
|
* `root=...`: If not set, importable pools are searched for a bootfs
|
|
attribute. If an explicitly set root is desired, you may use
|
|
`root=ZFS:pool/dataset`
|
|
|
|
* `zfs_force=0`: If set to 1, the initramfs will run `zpool import -f` when
|
|
attempting to import pools if the required pool isn't automatically imported
|
|
by the zfs module. This can save you a trip to a bootcd if hostid has
|
|
changed, but is dangerous and can lead to zpool corruption, particularly in
|
|
cases where storage is on a shared fabric such as iSCSI where multiple hosts
|
|
can access storage devices concurrently. _Please understand the implications
|
|
of force-importing a pool before enabling this option!_
|
|
|
|
* `spl_hostid`: By default, the hostid used by the SPL module is read from
|
|
/etc/hostid inside the initramfs. This file is placed there from the host
|
|
system when the initramfs is built which effectively ties the ramdisk to the
|
|
host which builds it. If a different hostid is desired, one may be set in
|
|
this attribute and will override any file present in the ramdisk. The
|
|
format should be hex exactly as found in the `/etc/hostid` file, IE
|
|
`spl_hostid=0x00bab10c`.
|
|
|
|
Note that changing the hostid between boots will most likely lead to an
|
|
un-importable pool since the last importing hostid won't match. In order
|
|
to recover from this, you may use the `zfs_force` option or boot from a
|
|
different filesystem and `zpool import -f` then `zpool export` the pool
|
|
before rebooting with the new hostid.
|
|
|
|
How it Works
|
|
============
|
|
|
|
The Dracut module consists of the following files (less Makefile's):
|
|
|
|
* `module-setup.sh`: Script run by the initramfs builder to create the
|
|
ramdisk. Contains instructions on which files are required by the modules
|
|
and z* programs. Also triggers inclusion of `/etc/hostid` and the zpool
|
|
cache. This file is not included in the initramfs.
|
|
|
|
* `90-zfs.rules`: udev rules which trigger loading of the ZFS modules at boot.
|
|
|
|
* `zfs-lib.sh`: Utility functions used by the other files.
|
|
|
|
* `parse-zfs.sh`: Run early in the initramfs boot process to parse kernel
|
|
command line and determine if ZFS is the active root filesystem.
|
|
|
|
* `mount-zfs.sh`: Run later in initramfs boot process after udev has settled
|
|
to mount the root dataset.
|
|
|
|
* `export-zfs.sh`: Run on shutdown after dracut has restored the initramfs
|
|
and pivoted to it, allowing for a clean unmount and export of the ZFS root.
|
|
|
|
`zfs-lib.sh`
|
|
------------
|
|
|
|
This file provides a few handy functions for working with ZFS. Those
|
|
functions are used by the `mount-zfs.sh` and `export-zfs.sh` files.
|
|
However, they could be used by any other file as well, as long as the file
|
|
sources `/lib/dracut-zfs-lib.sh`.
|
|
|
|
`module-setup.sh`
|
|
-----------------
|
|
|
|
This file is run by the Dracut script within the live system, not at boot
|
|
time. It's not included in the final initramfs. Functions in this script
|
|
describe which files are needed by ZFS at boot time.
|
|
|
|
Currently all the various z* and spl modules are included, a dependency is
|
|
asserted on udev-rules, and the various zfs, zpool, etc. helpers are included.
|
|
Dracut provides library functions which automatically gather the shared libs
|
|
necessary to run each of these binaries, so statically built binaries are
|
|
not required.
|
|
|
|
The zpool and zvol udev rules files are copied from where they are
|
|
installed by the ZFS build. __PACKAGERS TAKE NOTE__: If you move
|
|
`/etc/udev/rules/60-z*.rules`, you'll need to update this file to match.
|
|
|
|
Currently this file also includes `/etc/hostid` and `/etc/zfs/zpool.cache`
|
|
which means the generated ramdisk is specific to the host system which built
|
|
it. If a generic initramfs is required, it may be preferable to omit these
|
|
files and specify the `spl_hostid` from the boot loader instead.
|
|
|
|
`parse-zfs.sh`
|
|
--------------
|
|
|
|
Run during the cmdline phase of the initramfs boot process, this script
|
|
performs some basic sanity checks on kernel command line parameters to
|
|
determine if booting from ZFS is likely to be what is desired. Dracut
|
|
requires this script to adjust the `root` variable if required and to set
|
|
`rootok=1` if a mountable root filesystem is available. Unfortunately this
|
|
script must run before udev is settled and kernel modules are known to be
|
|
loaded, so accessing the zpool and zfs commands is unsafe.
|
|
|
|
If the root=ZFS... parameter is set on the command line, then it's at least
|
|
certain that ZFS is what is desired, though this script is unable to
|
|
determine if ZFS is in fact available. This script will alter the `root`
|
|
parameter to replace several historical forms of specifying the pool and
|
|
dataset name with the canonical form of `zfs:pool/dataset`.
|
|
|
|
If no root= parameter is set, the best this script can do is guess that
|
|
ZFS is desired. At present, no other known filesystems will work with no
|
|
root= parameter, though this might possibly interfere with using the
|
|
compiled-in default root in the kernel image. It's considered unlikely
|
|
that would ever be the case when an initramfs is in use, so this script
|
|
sets `root=zfs:AUTO` and hopes for the best.
|
|
|
|
Once the root=... (or lack thereof) parameter is parsed, a dummy symlink
|
|
is created from `/dev/root` -> `/dev/null` to satisfy parts of the Dracut
|
|
process which check for presence of a single root device node.
|
|
|
|
Finally, an initqueue/finished hook is registered which causes the initqueue
|
|
phase of Dracut to wait for `/dev/zfs` to become available before attempting
|
|
to mount anything.
|
|
|
|
`mount-zfs.sh`
|
|
--------------
|
|
|
|
This script is run after udev has settled and all tasks in the initqueue
|
|
have succeeded. This ensures that `/dev/zfs` is available and that the
|
|
various ZFS modules are successfully loaded. As it is now safe to call
|
|
zpool and friends, we can proceed to find the bootfs attribute if necessary.
|
|
|
|
If the root parameter was explicitly set on the command line, no parsing is
|
|
necessary. The list of imported pools is checked to see if the desired pool
|
|
is already imported. If it's not, and attempt is made to import the pool
|
|
explicitly, though no force is attempted. Finally the specified dataset
|
|
is mounted on `$NEWROOT`, first using the `-o zfsutil` option to handle
|
|
non-legacy mounts, then if that fails, without zfsutil to handle legacy
|
|
mount points.
|
|
|
|
If no root parameter was specified, this script attempts to find a pool with
|
|
its bootfs attribute set. First, already-imported pools are scanned and if
|
|
an appropriate pool is found, no additional pools are imported. If no pool
|
|
with bootfs is found, any additional pools in the system are imported with
|
|
`zpool import -N -a`, and the scan for bootfs is tried again. If no bootfs
|
|
is found with all pools imported, all pools are re-exported, and boot fails.
|
|
Assuming a bootfs is found, an attempt is made to mount it to `$NEWROOT`,
|
|
first with, then without the zfsutil option as above.
|
|
|
|
Ordinarily pools are imported _without_ the force option which may cause
|
|
boot to fail if the hostid has changed or a pool has been physically moved
|
|
between servers. The `zfs_force` kernel parameter is provided which when
|
|
set to `1` causes `zpool import` to be run with the `-f` flag. Forcing pool
|
|
import can lead to serious data corruption and loss of pools, so this option
|
|
should be used with extreme caution. Note that even with this flag set, if
|
|
the required zpool was auto-imported by the kernel module, no additional
|
|
`zpool import` commands are run, so nothing is forced.
|
|
|
|
`export-zfs.sh`
|
|
---------------
|
|
|
|
Normally the zpool containing the root dataset cannot be exported on
|
|
shutdown as it is still in use by the init process. To work around this,
|
|
Dracut is able to restore the initramfs on shutdown and pivot to it.
|
|
All remaining process are then running from a ramdisk, allowing for a
|
|
clean unmount and export of the ZFS root. The theory of operation is
|
|
described in detail in the [Dracut manual](https://www.kernel.org/pub/linux/utils/boot/dracut/dracut.html#_dracut_on_shutdown).
|
|
|
|
This script will try to export all remaining zpools after Dracut has
|
|
pivoted to the initramfs. If an initial regular export is not successful,
|
|
Dracut will call this script once more with the `final` option,
|
|
in which case a forceful export is attempted.
|
|
|
|
Other Dracut modules include similar shutdown scripts and Dracut
|
|
invokes these scripts round-robin until they succeed. In particular,
|
|
the `90dm` module installs a script which tries to close and remove
|
|
all device mapper targets. Thus, if there are ZVOLs containing
|
|
dm-crypt volumes or if the zpool itself is backed by a dm-crypt
|
|
volume, the shutdown scripts will try to untangle this.
|