[systemd-devel] btrfs raid not ready but systemd tries to mount it anyway

[Lennart Poettering]

On So, 11.10.20 14:57, Chris Murphy (lists at colorremedies.com) wrote:

> Hi,
>
> A Fedora 32 (systemd-245.8-2.fc32) user has a 10-drive Btrfs raid1 set
> to mount in /etc/fstab:
>
> UUID=f89f0a16-<snipped>  /srv   btrfs  defaults,nofail,x-systemd.requires=/  0 0
>
> For some reason, systemd is trying to mount this file system before
> all ten devices are ready. Supposedly this rule applies:
> https://github.com/systemd/systemd/blob/master/rules.d/64-btrfs.rules.in

udev calls the btrfs ready ioctl whenever a new btrfs fs block deice
shows up. The ioctl will fail as long as not all devices that make up
the fs have shown up. It succeeds once all devices for the fs are
there. i.e. for n=10 devices it will return failure 9 times, and
sucess the 1 final time.

When precisely it returns success or failure is entirely up to the btrfs kernel
code. systemd/udev doesn't have any control on that. The udev btrfs
builtin is too trivial for that: it just calls the ioctl and that
pretty much is it.

> x-systemd.automount,noauto,nofail,x-systemd.requires=/
>
> In fact, I'm not sure x-systemd.requires is needed because / must be
> mounted successfully to read /etc/fstab in the first place; in order
> to know to mount this file system at /srv

x-systemd.requires=/ is a NOP. the root fs is always mounted, in
systemd the unit "-.mount" that wraps it is marked with the
"perpetual" flag internally, which all units have set that are always,
unconditionally up as long as userspace exists. Thus requiring the
root fs is useless, it's always there and implicitly required by all
userspace.

Also note: in the initrd the host root fs is at /sysroot/ not at
/. During the initrd→host transition we do a switch root after all
that turns /sysroot/ → /.

> Anyway I'm mainly confused why the btrfs udev rule is seemingly not
> applied in this case.

For historical reasons udev log level is independent from the rest of
systemd log level. Thus use udev.log_priority=debug to turn on udev
debug logging.

Lennart

--
Lennart Poettering, Berlin