Re: [gentoo-amd64] Systemd migration: opinion and questions

[Rich Freeman <rich0@gentoo.org>]

On Wed, Feb 25, 2015 at 2:50 AM, Marc Joliet <marcec@gmx.de> wrote:
> Am Tue, 24 Feb 2015 16:44:59 -0500
> schrieb Rich Freeman <rich0@gentoo.org>:
>
>> > === Timers ===
>> >
>> > Can a systemd timer depend on a mount point such that it waits until the mount
>> > point exists before running?  Or will it fail after a timeout?  I want to
>> > research this myself, but haven't gotten around to it yet.
>>
>> So, timer units are units, and units can have dependencies, and mounts
>> can be dependencies since mounts are units.  However, if you set the
>> dependency on the timer itself, then the timer won't start running
>> until the mount exists.  You probably want the depencency to be on the
>> service started by the timer (so the timer is watching the clock, but
>> the service won't start without the mount).
>
> Wait, so the timer won't start watching the clock until its dependencies are
> met (i.e, the mount point appears)?  Is that what you mean?  Because that might
> be more in line with what I want (though I'm not sure yet).

If you set the dependency on the timer, then the timer doesn't start
watching the clock until they're met.  If you set the dependency on
the service started by the timer then it will watch the clock but not
launch the service if the dependency isn't met.  You can set the
dependency in either or both places.  The timer and the service are
both units.

>
>> If you set a
>> Requires=foo.mount and After=foo.mount, then the service shouldn't run
>> unless foo.mount is available.  I suspect systemd will attempt to
>> mount the filesystem when it runs the service, and you'll get units in
>> the failed state if that doesn't work.
>>
>> However, I haven't tested any of this.  I suspect it wouldn't take
>> much to work this out.  I have a mount dependency in one of my
>> services.  Just look at the mount units in /run/systemd/generator for
>> the name of the mount unit systemd is creating from fstab.
>
> Right, so IIUC, I would have a oneshot service that does the backup, and the timer
> runs that, and of course the timer can depend on the mount point.  And if the
> mount point doesn't exist, then the service started by the timer will fail.
>
> What I would prefer to have is a timer that only runs if *both* the time *and*
> mount conditions are met.  Skimming the man page, this does not seem possible.
> I suppose it would be nice if timers learned "conditions" on which they should
> wait in addition to the time condition, but maybe that's outside the scope of
> systemd?

I think if you just set the dependency on the service you'll get the
behavior you desire.  Systemd will try to mount the backup filesystem,
and if that fails it won't run the backup.

You can set conditions on units as well, like only running if they're
on AC power or on amd64 or to run one unit the first time you start a
service and a different unit every other time.  Some of that was
designed to implement some of the stateless system features they're
adding to systemd.

-- 
Rich