systemd and mounting filesystems

[Lennart Poettering]

On Tue, 04.10.11 14:39, Steven Whitehouse (swhiteho at redhat.com) wrote:

> Hi,

Heya,
> 
> I'm looking for some info on systemd and how filesystems are mounted in
> Fedora. I've started looking into converting the gfs2-utils package to
> the new init system and run into things which are not documented (so far
> as I can tell).
> 
> Currently there are two init scripts in gfs2-utils, one is called gfs2
> and the other gfs2-cluster.
> 
> Converting gfs2-cluster is trivial. It simply runs the gfs_controld
> daemon on boot.
> 
> The more complicated conversion is the gfs2 script. This has been used
> historically to mount gfs2 filesystems (rather than using the system
> scripts for this). I assume that under the new systemd regime it should
> be possible to simply tell systemd that gfs2 filesystem mounting
> requires gfs_controld to be running in addition to the normal filesystem
> requirement of having the mount point accessible, and then systemd would
> do the mounting itself.

systemd will automatically order all network mounts after
network.target. It recognizes network mounts either by "_netdev" in the
options field in fstab, or by the file system type (it has a short
static list of known network file systems built in, and gfs2 is actually
listed in it).

systemd automatically orders mounts by their path. i.e. /foo will always
be mounted before /foo/bar.

So, probably you should simply order gfs2-cluster before network.target
and that's already all you need to do:

[Unit]
Before=network.target

> Things are slightly more complicated in that gfs_controld is only a
> requirement for gfs2 when lock_dlm is in use. For lock_nolock
> filesystems, mounting is just like any other local filesystem. The
> locking type can be specified either in fstab, or in the superblock
> (with fstab taking priority).

Well, I'd probably recommend to just ask people to enable gfs_controld
manually with "systemctl enable" if they want to make use of it. But if
you want an automatic pulling in depending on the mount option you could
write a generator. That's a tiny binary (or script) you place in
/lib/systemd/system-generators/. It will be executed very very early at
boot and could generate the necessary deps by parsing fstab and creating
.wants symlinks in the directory the generator gets passes as
argv[1]. This is fairly simple to do, but I am tempted to say that
manually enabling this service is nicer in this case. Automatisms in
some areas are good but manually enabling the service is sometimes an
option too. There's little documentation available on generators right
now, simply because we don't want to advertise them too widely yet, and
prefer if people ping us if they plan to make use of it in some package.

> Another issue which I suspect is already resolved, but I'm not quite
> sure how it can be specified in fstab, etc, is that of mount order of
> filesystems. In particular how to set up bind mounts such that they
> occur either before or after a specified filesystem?

systemd should be smart enought to handle that automatically. For bind
mounts we wait until all mount points that are prefixes of either the
mount source or the mount destination are mounted before we apply the
bind mounts.

Lennart

-- 
Lennart Poettering - Red Hat, Inc.