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

[Patrick O'Callaghan]

On Fri, 2021-03-12 at 23:00 +0100, Silvio Knizek wrote:
> Am Donnerstag, dem 11.03.2021 um 11:00 +0000 schrieb 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
> Hi,
> 
> I don't really understand your problem. Do you want to unmount the
> backup drive whenever it's not in use? There is the mount option x-
> systemd.automount,x-systemd.idle-timeout=5s (man systemd.mount) to
> mount/unmount on demand.

That's not the issue. I know I can automount/unmount and this is
already working. However I also want to power down the external drive
after unmounting, and power it up again before mounting. I have my own
script to handle the power up/down and that also works. What I haven't
managed to do is join those two elements together.

> Can you share your fstab entry for your backup device?

UUID=6cb66da2-147a-4f3c-a513-36f6164ab581      /raid                   ext4    rw,noauto,user,x-systemd.automount 0 0

As I said, this in itself works fine, as long as I leave the drive
permanently powered on.

poc