Until now nixos only delivered the latest zfs release. This release is often not
compatible with the latest mainline kernel. Therefor an unstable variant is
added, which might be based on testing releases or git revisions.
fixes#21359
The most complex problems were from dealing with switches reverted in
the meantime (gcc5, gmp6, ncurses6).
It's likely that darwin is (still) broken nontrivially.
If you power off your machine frequently, you could miss the execution
of some snapshots.
This is more troublesome the more infrequently the snapshots are
triggered. For example, monthly snapshots only execute at exactly
midnight on the first day of the month. If you only have your
machine powered on at that time with probability 50%, then half the
snapshots won't be triggered.
This means that if you wanted to keep 3 monthly snapshots, then instead
of keeping 3 months' worth of snapshotted data as you expected, you would
end up with snapshots spanning back 6 months.
Adding the "Persistent = yes" option to auto-snapshot timer units makes
a missed snapshot execute when booting up the machine.
Otherwise, in certain cases, snapshots of infrequently-modified
filesystems can be kept for a much longer time than the user would
normally expect, and cause a large amount of extra disk space to be
consumed.
Also added flag to snapshot filesystems in parallel by default.
I've also added a configuration option for zfs-auto-snapshot flags, so
that the user can override them.
For example, the user may want to append --utc to the list of default
options, so that the snapshot names don't cause name conflicts or
apparent time reversals due to daylight savings or timezone changes.
The old boot.spl.hostid option was not working correctly due to an
upstream bug.
Instead, now we will create the /etc/hostid file so that all applications
(including the ZFS kernel modules, ZFS user-space applications and other
unrelated programs) pick-up the same system-wide host id. Note that glibc
(and by extension, the `hostid` program) also respect the host id configured in
/etc/hostid, if it exists.
The hostid option is now mandatory when using ZFS because otherwise, ZFS will
require you to force-import your ZFS pools if you want to use them, which is
undesirable because it disables some of the checks that ZFS does to make sure it
is safe to import a ZFS pool.
The /etc/hostid file must also exist when booting the initrd, before the SPL
kernel module is loaded, so that ZFS picks up the hostid correctly.
The complexity in creating the /etc/hostid file is due to having to
write the host ID as a 32-bit binary value, taking into account the
endianness of the machine, while using only shell commands and/or simple
utilities (to avoid exploding the size of the initrd).
