Bug#947806: systemd doesn't like my raid, times out waiting for online partitions to come online, and can't continue boot

[Joshua Hudson]

That turned out to be really easy to track down. The reason you aren't
getting any notification of the raid array being activated is it was
already activated in initrd. It will not be activated a second time.

dmraid has some interesting principles in play here.

1) dmraid has its own idea of persistent, path-independent names of devices.
2) dmraid is incompatible with UUID= or LABEL=; all users of dmraid
must have the raid device names in /etc/fstab and on the kernel
command line. GRUB_LINUX_DISABLE_UUID *must* be used.
3) Under normal operation, dmraid is included in initrd as soon as it
is installed on the system.
4) If dmraid is installed in initrd, udev will discover the dmraid
devices before / is mounted. If udev discovers the dmraid devices, it
will start the system.
5) Redundant activations are not possible.

So the effect of all this is really simple. systemd can just assume
that a dmraid device under /dev/mapper is active if the device node
exists at all, and on any reasonable system it doesn't even need to
poll the device path; it can check *once* and know the success or
failure immediately.