RHCSA – Automounting using systemd and autofs

Overview

By the end of this article you should be able to answer the following questions:

Let's say we have the following scenario:

• we have the partition: /dev/sdb1
• we have the mountpoint: /tmp/sdb1custom
• the /dev/sdb1 partition has the xfs mountpoint installed on it.
• The /dev/sdb1 needs to be automounted by the multi-user.target

Earlier when we covered autofs, we found how a filesystem is not mounted until you actually cd into it's mount point.

You can achieve the same thing with systemd as well. To get started, you first need to create a mountpoint and a *.mount file, like we did in the previous lesson, so that it appears that we are going to auto-mount the file system during machine boot. But this time we don't start the mount "unit" and we ensure that it is disabled.

Let's use the filesystem, sdb1, from the last lesson as an example. So we have our filesystem:

$ ls -l /dev/sdb1
brw-rw----. 1 root disk 8, 17 Jun  9 06:12 /dev/sdb1

...and we have our mountpoint:

$ ls -l /tmp | grep sdb1
drwxr-xr-x. 2 root root  6 Jun  8 21:01 sdb1custom

...and we have our *.mount file:

$ cat /etc/systemd/system/tmp-sdb1custom.mount
[Unit]
Description=My new file system

[Mount]
What=/dev/sdb1
Where=/tmp/sdb1custom
Type=xfs
Options=grpquota,ro

[Install]
WantedBy=multi-user.target

Next in order to take the automount route, we actually need to stop+disable this unit:

$ systemctl stop tmp-sdb1custom.mount
$ systemctl disable tmp-sdb1custom.mount
rm '/etc/systemd/system/multi-user.target.wants/tmp-sdb1custom.mount'

So far so good. Now we come to the steps we need to take to set up the actual automounting feature. To do this we create a new *.automount unit file in the /etc/systemd/system directory. This file must have the same name as the corresponding *.mount file, but with a ".automount" suffix. Therefore it's easier to create this file by making it a copy of the .mount file:

$ cp /etc/systemd/system/tmp-sdb1custom.mount /etc/systemd/system/tmp-sdb1custom.automount
$ ls -l /etc/systemd/system/tmp-sdb1custom.automount
-rw-r--r--. 1 root root 151 Jun  9 06:30 /etc/systemd/system/tmp-sdb1custom.automount

Now you need to edit this file so that it looks like this:

$ cat /etc/systemd/system/tmp-sdb1custom.automount
[Unit]
Description=My new file system
Description=My new automounted file system


[Mount]
What=/dev/sdb1
Where=/tmp/sdb1custom
Type=xfs
Options=grpquota,ro


[Automount]
Where=/tmp/sdb1custom

[Install]
WantedBy=multi-user.target

Notice, that this unit's main config section is now called "[Automount]". Also notice that I included a "Where" setting in this section. This is actually optional, and if specified, it must match what is in the corresponding .mount file. It's doesn't work like some kind of over-ride mechanism, as you might have initially suspected, you will get an error message if you tried.

Now let's enable and start this unit:

$ systemctl status tmp-sdb1custom.automount
tmp-sdb1custom.automount - My new automounted file system
   Loaded: loaded (/etc/systemd/system/tmp-sdb1custom.automount; disabled)
   Active: inactive (dead)
    Where: /tmp/sdb1custom

$ systemctl start tmp-sdb1custom.automount
$ systemctl enable tmp-sdb1custom.automount
ln -s '/etc/systemd/system/tmp-sdb1custom.automount' '/etc/systemd/system/multi-user.target.wants/tmp-sdb1custom.automount'
$ systemctl status tmp-sdb1custom.automount
tmp-sdb1custom.automount - My new automounted file system
   Loaded: loaded (/etc/systemd/system/tmp-sdb1custom.automount; enabled)
   Active: active (waiting) since Tue 2015-06-09 06:48:00 BST; 11s ago
    Where: /tmp/sdb1custom

Jun 09 06:48:00 localhost.localdomain systemd[1]: Starting My new automounted file system.
Jun 09 06:48:00 localhost.localdomain systemd[1]: Set up automount My new automounted file system.

Once it has been started, you will see that the mount command shows that it is now a auto-mounting (aka autofs) mountpoint:

$ mount | grep sdb1
systemd-1 on /tmp/sdb1custom type autofs (rw,relatime,fd=45,pgrp=1,timeout=300,minproto=5,maxproto=5,direct)

However lsblk shows that it hasn't been mounted yet:

$ lsblk
NAME            MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda               8:0    0   20G  0 disk
├─sda1            8:1    0  500M  0 part /boot
└─sda2            8:2    0 19.5G  0 part
  ├─centos-swap 253:0    0    2G  0 lvm  [SWAP]
  └─centos-root 253:1    0 17.5G  0 lvm  /
sdb               8:16   0    2G  0 disk
└─sdb1            8:17   0    1G  0 part
sr0              11:0    1 1024M  0 rom

Let's now cd into the mountpoint and try again:

$ cd /tmp/sdb1custom/

Then try lsblk again:

$ lsblk
NAME            MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda               8:0    0   20G  0 disk
├─sda1            8:1    0  500M  0 part /boot
└─sda2            8:2    0 19.5G  0 part
  ├─centos-swap 253:0    0    2G  0 lvm  [SWAP]
  └─centos-root 253:1    0 17.5G  0 lvm  /
sdb               8:16   0    2G  0 disk
└─sdb1            8:17   0    1G  0 part /tmp/sdb1custom
sr0              11:0    1 1024M  0 rom

Success!

Also the mount command also shows that it has been mounted:

$ mount | grep sdb1
systemd-1 on /tmp/sdb1custom type autofs (rw,relatime,fd=45,pgrp=1,timeout=300,minproto=5,maxproto=5,direct)
/dev/sdb1 on /tmp/sdb1custom type xfs (ro,relatime,seclabel,attr2,inode64,grpquota)