[systemd-devel] udev and btrfs multiple devices

[Lennart Poettering]

On Mi, 27.01.21 17:19, Chris Murphy (lists at colorremedies.com) wrote:

> Is it possible for a udev rule to have a timeout? For example:
> /usr/lib/udev/rules.d/64-btrfs.rules
>
> This udev rule will wait indefinitely for a missing device to
> appear.

Hmm, no, that's a mis understaning. "rules" can't "wait". The
activation of the btrfs file system won't happen, but that should then
be caught by systemd mount timeouts and put you into recovery mode.

udev only does the most basic form of btrfs raid activation. If you
want more complex policies (i.e. "boot cleanly if 5 of the 7 hdd's
have shown up plus 30s passed") then put that into some storage
daemon. udev is not supposed to implement complex policies like that.

> It'd be better if it gives up at some point and drops to a dracut
> shell. Is that possible? The only alternative right now is the user
> has to force power off, and boot with something like
> rd.break=pre-mount, although I'm not 100% certain that'll break soon
> enough to avoid the hang.

That should be the default.

But note that once you throw cryptsetup into mix the mount activation
timeout is typically automatically turned off, because we cannot
distinguish the cases where devices didn't show up because hdd borked
from "user didn't type in their pw in time". To give the user any time
they want for typing it in the logic thus turns off root fs activation
timeouts.

> Next, is it possible to enhance udev so that it can report the number
> of devices expected for a Btrfs file system? This information is
> currently in the Btrfs superblock found on each device in the
> num_devices field.
> https://github.com/storaged-project/udisks/pull/838#issuecomment-768372627

systemd/udev has no clue about btrfs superblocks. It only talks to
libblkid, and to the kernel. It doesn't parase anything on its own. If
you want this property to be attached to the udev device, please
request this from the libblkid maintainer, since we basically import
all props it gives us 1:1 into udev rules.

Lennart

--
Lennart Poettering, Berlin