[systemd-devel] Starting a service on shutdown

[Lennart Poettering]

On Mon, 21.01.13 13:38, Umut Tezduyar (umut at tezduyar.com) wrote:

> I would like to start a service on shutdown/restart. This service accesses
> (read/write) to the file systems and for that reason I don't want any of
> the file systems unmounted by systemd before the service completes.

Right now when you start a service, and you stop another at the same
time and they have an ordering dependency (regardless in which
direction), then this will cause the stop to be executed first, and the
start to be executed second. Currently, this is all you can express with
systemd units: you are either not ordered at all or the "stop" is before
the "start". That's certainly a limitation, and we have a todo list item
to come up with a good, convincing solution for this problem. But so
far, we didn't.

In the meantime, if you want to start a service at shutdown before other
units go down, then one possible solution is to just create a service
that is started at bootup, does nothing in ExecStart= and has all
interesting bits in ExecStop=. With Type=oneshot, RemainAfterExit=yes it
would then nominally stay around during the entire runtime but without
any processes or anything else. Then, order this service after
local-fs,target and remote-fs.target, and at shutdown it will be run
before all mounts go away.

This solution should work pretty well. It isn't perfect though, there
are probably cases this doesn't cover (though so far we could cover all
cases that came up with this scheme, but maybe I missed
something?). Also, currently you must specify ExecStart= in a service,
even if all you are interested in is ExecStop=. That's a limitation we
really should lift though (it's easy, and it's on the todo list...), but
in the meantime just invoke /bin/true in ExecStart= and you should be
good.

Hope that makes some sense.

Lennart

-- 
Lennart Poettering - Red Hat, Inc.