Bug#762984: Alert! /dev/vg0/usr does not exist

[Simon McVittie]

On Wed, 01 Oct 2014 at 22:18:53 +0100, Ben Hutchings wrote:
> I suspect this is essentially the same bug as #616689 and #678696,
> except that now it may affect mounting /usr as well as /.

I think this bug report is actually describing more than one bug in more
than one package that have similar symptoms. There might be things
that can be fixed in mdadm and lvm2 to fix the initramfs-tools/0.117
regressions without needing to implement a full event-driven setup in
initramfs-tools.

---- RAID (Elimar, Sven) ----

Elimar Riesebieter's "System 2" has a bunch of mdadm (RAID) partitions.

Elimar, what is in your /etc/default/mdadm on "System 2" (and "System 1"
for that matter)? I predict that the answer includes something like
"INITRDSTART=/dev/md6".

The problem here seems to be that mdadm tries to determine a minimal
set of multi-disk partitions need to be assembled by the initramfs
based on the assumption that the initramfs only needs the root device;
but initramfs-tools >= 0.117 wants to mount /usr as well, so that
assumption is no longer true.

So it might be necessary to modify mdadm so that, if /usr is a separate
filesystem on (a LVM VG on) a MD array, it will try to prepare that too.

---- LVM (Elimar's "System 1", Sven, Torsten, IOhannes, Javier) ----

In the LVM case, debian/initramfs-tools/lvm2/scripts/local-top
does this:

    activate_vg "$ROOT"
    activate_vg "$resume"

Note the lack of handling for /usr here.

Further, activate_vg uses "lvm lvchange" to activate only the LV
necessary for the root filesystem; if /usr is on a separate VG,
it's not going to work.

This on its own would be enough to make Sven Hartge's system fail:
/usr needs a LVM partition activated that / does not. Similarly,
I think Elimar's "System 1" is going to activate vg0/root but not
vg0/usr.

We don't have enough information in this bug report to know what
layout Torsten, IOhannes, Javier used on their systems, but we can
guess from the fact that "vgchange -a y" is a successful workaround...
I predict that these are LVM over either a single RAID array, or
real partitions.

--------

The ideal thing in both of these situations would be to use the same
logic as *mounting* /usr - mount the rootfs first, then read its fstab
to find out where /usr is, avoiding hard-coding that knowledge into
the initramfs - but I think that would need a significantly more
complicated hook structure.

Perhaps modifying mdadm and lvm2 so they will set up enough md/lvm bits
for /usr in the initramfs would be sufficient for a 95% solution?

    S