[systemd-devel] Need help with a systemd/mdadm interaction.

[Jan Engelhardt]

On Tuesday 2013-11-12 01:31, NeilBrown wrote:
>
>mdadm is quite good at assembling arrays incrementally.  "udev" runs
>"mdadm -I" for each new device and mdadm gathers them into arrays and
>activates the array once all the expected devices have been gathered.
>[mdadm might] wait [...] forever.
>mdadm can handle this, but it needs to be told.  The command:
>  mdadm -IRs
>will find any arrays which have enough devices to start degraded but haven't
>been started yet, and will start them.

Using mdadm -R in udev would be a really bad idea, because it would
make for a race, I think. Consider a system that has finished booting
and is as ready as can be, and you plug in USB disks that form a RAID
array. If you use -R, the array may start before you had the chance
to plug them all in, causing the array to start in degraded mode,
potentially causing a long resync when all missing devices have been
plugged into their sockets.

>Alternately, is there some "all devices have been probed, nothing new will
>appear unless it is hot-plugged" event.  That would be equally useful (and
>probably mirrors what hardware-RAID cards do).

IIRC, openSUSE had a /etc/init.d/boot.coldplug once upon a time                 
(seems to be 9.3) that acted like what you describe.
The order was like udevadm trigger, load static module list,
udevadm settle, run mdadm, chroot /realroot, repeat for real root
once more. Would that still work?