Mount vs automount systemd units: which one to use for what?

Question

After doing some research about systemd units, I found two different kinds of mount unit: the .mount and the .automount. At first, it seemed logical to me that the automount unit would mount a mount automatically. However, as it turns out, when you enable a mount unit:

systemctl enable media-mydisk.mount

It will automatically be mounted at boot. I'm somewhat new to systemd, but this has been bugging me for quite some time already. I've also posted the code of the unit at the end.

So, my main question is: why does one need automounts if one can just enable a mount unit?

Here's my media-mydisk.mount if that makes any difference:

[Unit]
Description=My disk

[Mount]
What=/dev/sdb1
Where=/media/mydisk
Type=ext4
Options=defaults

[Install]
WantedBy=multi-user.target

I've searched the web but couldn't find any comparison between using an .automount and simply enabling a .mount

Answer 1

The "auto" part in automount does not refer to the boot process: automount units define mount points that are mounted on-demand, i.e. only when they are accessed.

automount units are optional; but, when they exist, corresponding mount units must also exist. The former are meant to add functionalities to existing instances of the latter. From man systemd.mount:

Optionally, a mount unit may be accompanied by an automount unit, to allow on-demand or parallelized mounting.

And, from man systemd.automount:

For each automount unit file a matching mount unit file (see systemd.mount(5) for details) must exist which is activated when the automount path is accessed. Example: if an automount unit home-lennart.automount is active and the user accesses /home/lennart the mount unit home-lennart.mount will be activated.

A typical use case for automount units is mounting file systems (e.g. on remote or removable or encrypted media) that are not required during the boot process and may slow it down, or that may be unavailable at boot, but that you still like having managed by systemd.

A simple, illustrative-only example. Given the mnt-foo.mount unit

[Unit]
Description=foo mount

[Mount]
Where=/mnt/foo
What=/home/user/foo
Type=ext4

(for simplicity, foo is just a regular file formatted as ext4), and the mnt-foo.automount unit

[Unit]
Description=foo automount

[Automount]
Where=/mnt/foo

[Install]
WantedBy=multi-user.target

after the latter is activated (or enabled, and the system rebooted)

# systemctl start mnt-foo.automount

you will be able to check that /home/user/foo is not mounted anywhere yet—mount gives

$ mount | grep foo
systemd-1 on /mnt/foo type autofs (...)

and indeed /home/user/foo is only mounted on /mnt/foo as soon as you access the mount point:

$ ls /mnt/foo

$ mount | grep foo
systemd-1 on /mnt/foo type autofs (...)
/home/user/foo on /mnt/foo type ext4 (rw,relatime)