Bind mounts with systemd and non-fstab filesystems

August 13, 2014

Under normal circumstances the way you deal with Linux bind mounts on a systemd based system is the same as always: you put them in /etc/fstab and systemd makes everything work just like normal. If you can deal with your bind mounts this way, I recommend that you do it and keep your life simple. But sometimes life is not simple.

Suppose, not entirely hypothetically, that you are dealing with base filesystems that aren't represented in /etc/fstab for one reason or another; instead they appear through other mechanisms. For example, perhaps they appear when you import a ZFS pool. You want to use these filesystems as the source of bind mounts.

The first thing that doesn't work is leaving your bind mounts in /etc/fstab. There is no way to tell systemd to not create them until something else happens (eg your zfs-mount.service systemd unit finishes or their source directory appears), so this is basically never going to do the right thing. If you get bind mounts at all they are almost certainly not going to be bound to what you want. At this point you might be tempted to think 'oh, systemd makes /etc/fstab mounts into magic <name>.mount systemd units, I can just put files in /etc/systemd/system to add some extra dependencies to those magic units'. Sadly this doesn't work; the moment you have a real <name>.mount unit file it entirely replaces the information from /etc/fstab and systemd will tell you that your <name>.mount file is invalid because it doesn't specify what to mount.

In short, you need real .mount units for your bind mounts. You also need to force the ordering, and here again we run into something that would be nice but doesn't work. If you run 'systemctl list-units -t mount', you will see that there are units for all of your additional non-fstab mounts. It's tempting to make your bind mount unit depend on an appropriate mount unit for its source filesystem, eg if you have a bind mount from /archive/something you'd have it depend on archive.mount. Unfortunately this doesn't work reliably because systemd doesn't actually know about these synthetic mount units before the mount appears. Instead you can only depend on whatever .service unit actually does the mounting, such as zfs-mount.service.

(In an extreme situation you could create a service unit that just used a script to wait for the mounts to come up. With a Type=oneshot service unit, systemd won't consider the service successful until the script exits.)

The maximally paranoid set of dependencies and guards is something like this:


(This is for a bind mount from /local/var/local to /var/local.)

We can't use a RequiresMountsFor on /local/var, because as far as systemd is concerned it's on the root filesystem and so the dependency would be satisfied almost immediately. I don't think the Condition will cause systemd to wait for /local/var/local to appear, just stop the bind mount from trying to be done if ZFS mounts happened but they didn't managed to mount a /local/var for some reason (eg a broken or missing ZFS pool).

(Since my /var is actually on the root filesystem, the RequiresMountsFor is likely gilding the lily; I don't think there's any situation where this unit can even be considered before the root filesystem is mounted. But if it's a separate filesystem you definitely want this and so it's probably a good habit in general.)

I haven't tested using local-var.mount in just the Requires here but I'd expect it to fail for the same reason that it definitely doesn't work reliably in an After. This is kind of a pity, but there you go and the Condition is probably good enough.

(If you don't want to make a bunch of .mount files, one for each mount, you could make a single .service unit that has all of the necessary dependencies and runs appropriate commands to do the bind mounting (either directly or by running a script). If you do this, don't forget to have ExecStop stuff to also do the unmounts.)

Sidebar: the likely non-masochistic way to do this for ZFS on Linux

If I was less stubborn, I would have set all of my ZFS filesystems to have 'mountpoint=legacy' and then explicitly mentioned and mounted them in /etc/fstab. Assuming that it worked (ie that systemd didn't try to do the mounts before the ZFS pool came up), this would have let me keep the bind mounts in fstab too and avoided this whole mess.

Written on 13 August 2014.
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Last modified: Wed Aug 13 23:19:00 2014
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