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Mounting Partitions Using systemd


systemd is gradually becoming the de facto init system & service manager replacing the old sysV init scripts & upstart. Recently, I discovered you can mount partitions using systemd.mount by writing your own .mount systemd unit file.

super suprised

After RTFM'ing, I realized, under the hood, systemd just runs mount command to mount the specified partition with the specified mount options listed in the mount unit file. Basically, you need to specify the following options in your unit file:

  • What= a partition name, path or UUID to mount
  • Where= an absolute path of a directory i.e. path to a mount point. If the mount point is non-existent, it will be created
  • Type= file system type. In most cases mount command auto-detects the file system
  • Options= Mount options to use when mounting

In the end, you can convert your typical fstab entry such as this:

UUID=86fef3b2-bdc9-47fa-bbb1-4e528a89d222 /mnt/backups    ext4    defaults      0 0

to:

[Mount]
What=/dev/disk/by-uuid/86fef3b2-bdc9-47fa-bbb1-4e528a89d222
Where=/mnt/backups
Type=ext4
Options=defaults

I Got This!

So I wrote a simple systemd mount unit file — /etc/systemd/system/mnt-backups.mount — which didn't work at first because I fell victim to one of the systemd.mount pitfalls:

Mount units must be named after the mount point directories they control. Example: the mount point /home/lennart must be configured in a unit file home-lennart.mount.

Huh? Yes that's right! The unit filename should match the mount point path.

mnt-backups.mount mount unit file:

[Unit]
Description=Mount System Backups Directory

[Mount]
What=/dev/disk/by-uuid/86fef3b2-bdc9-47fa-bbb1-4e528a89d222
Where=/mnt/backups
Type=ext4
Options=defaults

Reload systemd daemon & start the unit.

systemctl daemon-reload
systemctl start mnt-backups.mount

And just like any other unit, you can view its status using systemctl status mnt-backups.mount:

root@vast ~ # systemctl status mnt-backups.mount
 mnt-backups.mount - Mount System Backups Directory
   Loaded: loaded (/etc/systemd/system/mnt-backups.mount; enabled; vendor preset: disabled)
   Active: active (mounted) since Mon 2015-08-31 08:09:15 EAT; 2 days ago
    Where: /mnt/backups
     What: /dev/sdc
  Process: 744 ExecMount=/bin/mount /dev/disk/by-uuid/86fef3b2-bdc9-47fa-bbb1-4e528a89d222 /mnt/backups -n -t ext4 -o defaults (code=exited, status=0/SUCCESS)

Aug 31 08:09:15 vast systemd[1]: Mounting Mount System Backups Directory...
Aug 31 08:09:15 vast systemd[1]: Mounted Mount System Backups Directory.

Gotchas!!

After a reboot, I noticed the unit wasn't started & as result the mount point dir. was empty. The unit file was missing an [Install] section which contains installation information such as unit dependencies(WantedBy=, RequiredBy=), aliases(Alias=), additional units(Also=), e.t.c for the specified unit. In this case, I set the unit to start in multi-user runlevel a.k.a multi-user.target. Oh, did you know you can change runlevel using systemctl isolate $RUN_LEVEL.target? Read more about systemd runlevels/targets.

Here's the complete /etc/systemd/system/mnt-backups.mount unit file with an [Install] section:

[Unit]
Description=Mount System Backups Directory

[Mount]
What=/dev/disk/by-uuid/86fef3b2-bdc9-47fa-bbb1-4e528a89d222
Where=/mnt/backups
Type=ext4
Options=defaults

[Install]
WantedBy=multi-user.target

As always, enable the unit to start automatically during boot.

systemctl enable mnt-backups.mount