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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

ummw
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1 Answers1

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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)
fra-san
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  • Thanks for clarifying it to me! Alright, one more question: you mentioned one can use an automount for drives > that are not required during the boot process Does this mean that when `enable`ing a drive, it _must_ be mounted for the system to boot correctly? Or does this depend on the target or unit config? – ummw Mar 05 '20 at 08:00
  • @Linux4win Sorry for reposting, my previous comments had several typos. Not sure I'm understanding your question, but: 1) regardless of `automount` units, enabled `mount` units are only mounted at boot if they are needed (e.g. `WantedBy`) in order to reach the default target (in systemd parlance); if so, and if their resource is not available, 1.1) the system won't boot if they hold a needed part of the tree (e.g. `/`); 1.2) otherwise, their mount operation will fail, possibly slowing down the boot process. (cont.) – fra-san Mar 05 '20 at 22:30
  • (cont.) 2) A file system in an enabled _and activated_ (started) `automount` unit only needs to be available when accessed; it doesn't need to be available at boot. If an `automount` unit is not started, its file system is not mounted when the corresponding mount point is accessed. 3) Having both an enabled _and activated_ `mount` unit and a corresponding enabled and activated `automount` unit at boot makes only sense in corner cases that are beyond the scope of this answer, I guess (e.g. mounting a file system at boot and unmounting it when not used by means of the `TimeoutIdleSec` option). – fra-san Mar 05 '20 at 22:31
  • I guess you would have to add a "nofail" to your options, since it is mount which will mount. The you can enable without fearing not being able to boot up. Now honor the effort and accept the answer. @Linux4Win – WGRM Mar 06 '20 at 18:04