Explaining to systemd that /var is symlinked to /home/var

Question

Suppose an installation of Debian unstable, using systemd for init, with two filesystem partitions, / and /home. Suppose further that, for reasons to do with the physical disks, I have moved the contents of /var to /home/var and replaced the /var directory with an appropriate symlink. (Please do not try to talk me out of moving /var into the /home partition, or turn this into a systemd argument ;-)

With this configuration, it is necessary to inform systemd that any unit which requires anything in /var cannot be started until after /home is mounted. The one I know to be broken (as it tries to access a file in /var/lib very early in the boot sequence) is systemd-random-seed.service, but there could easily be any number of others that I haven't happened to notice yet.

What is the best way to configure the general rule that "anything needing something from /var cannot be started until after /home is mounted"? I will accept an answer of the form "add Requires= and After= directives to every single unit file affected" only if you can demonstrate that there is no superior alternative.

The version of systemd currently in Debian unstable is 224.

Answer 1

Well, init cannot really know all by itself which files a given service needs, so "this service uses /var" dependencies still need to be declared somewhere anyway.

Of course, that ought to be done by the developers & packagers, not by you. For example, the aforementioned systemd-random-seed.service already has all the necessary dependencies:

$ systemctl cat systemd-random-seed
# /usr/lib/systemd/system/systemd-random-seed.service
#  This file is part of systemd.
...
DefaultDependencies=no
RequiresMountsFor=/var/lib/systemd/random-seed
...

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So in your case, the "superior alternative" is to use a bind mount in place of the symlink. That will hook naturally into systemd's .mount unit dependencies while providing identical functionality to a symlink otherwise.

That is, if you have a bind mount on /var, then all units which already depend on var.mount will automatically (indirectly) depend on home.mount.

# /etc/fstab
/home/var  /var  none  bind  0  0

(If that's not acceptable, perhaps compiling a custom systemd version with the dependency hacked in would fulfill your 'requirements' better.)

---

If some of your .service units lack proper dependencies, there is another option – you can turn /var into an automount using systemd's autofs4 support.

With automounts, any process trying to access files under /var will block until the filesystem is mounted. So a global 'dependency' is created without having to edit individual service units.

To do this, add the x-systemd.automount option into fstab. (Or, if you prefer var.mount over fstab, then create a corresponding var.automount as well.)

# /etc/fstab
/home/var  /var  none  bind,x-systemd.automount  0  0

Of course this again requires that /var be a bind mount rather than a symlink.

Answer 2

More than a year later with the version of systemd (229)now shipping with ubuntu 16.04 there is support in fstab for dependency mounting like this.

so it's as easy as doing this.

# /etc/fstab
home/var /var x-systemd.requires=/home,x-systemd.automount,none bind 0 0

got the idea from this post https://copyninja.info/blog/systemd_automount_entry.html