[systemd-devel] Guidance on automount/unmount for external powered devices

[Patrick O'Callaghan]

Apologies in advance if this is a FAQ, or if there is a more
appropriate list for this question. I'm looking for a step-by-step
guide for the following situation:

I have an external 2-bay USB3 drive enclosure, configured as an MD
Raid-1 device. This works without issue.

I normally only use the device for nightly backups, so would prefer to
leave it powered off until needed.

I have scripts to power it up (creating the appropriate /dev/md entry
and mounting the drive) and down (unmounting, removing the /dev/md
entry and sending a magic signal to the USB device so it powers itself
down after 30 minutes).

Unfortunately a system reboot always leaves it in the 'powered up and
present in /dev/md' state, so I have to manually run the power-down
script every time I reboot. This is inelegant.

I would much prefer to have this all handled automagically by systemd,
but I need guidance on how to configure it. If it weren't for the power
question, I know I can just use automount (and have managed to get this
far on my own), but I don't know where or how to insert the power
scripts. My reading of the systemd docs leads me to think that
ExecStart/Stop might be the way, but where do I put this? Do I need a
specific foo.mount unit (I'm currently using /etc/fstab)?

poc