5

I mount /tmp on tmpfs using:

sudo systemctl enable tmp.mount
sudo systemctl start tmp.mount

But this way /tmp takes up all the free RAM:

$ df -h /tmp
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           3.9G   12K  3.9G   1% /tmp
$

How do I tell systemd tmp.mount to use only 1G? I know I can alternatively not use systemd and manually add an entry to /etc/fstab and specify the size there. But I don't want to do that. I want to use systemd backed tmpfs.

5

The systemd way of overriding tmp.mount, or extending it, is to add a local override in /etc/systemd/system. You can either copy the existing tmp.mount (from /lib/systemd/system or /usr/share/systemd probably) and edit the copy, or better yet, add configuration snippets to only change the mount options:

  • create a directory called /etc/systemd/system/tmp.mount.d
  • inside that directory, add a file called options.conf containing

    [Mount]
    Options=mode=1777,strictatime,nosuid,nodev,size=1073741824
    

Note that systemd.mount still says that

In general, configuring mount points through /etc/fstab is the preferred approach.

so you may just want to do that. In fact, this is the recommended approach to change mount options for any of systemd’s “API file systems”:

Even though normally none of these API file systems are listed in /etc/fstab they may be added there. If so, any options specified therein will be applied to that specific API file system. Hence: to alter the mount options or other parameters of these file systems, simply add them to /etc/fstab with the appropriate settings and you are done. Using this technique it is possible to change the source, type of a file system in addition to simply changing mount options. That is useful to turn /tmp to a true file system backed by a physical disk.

API file systems include the following: /sys, /proc, /dev, /run, /tmp, /sys/fs/cgroup, /sys/kernel/security, /sys/kernel/debug, /sys/kernel/config, /sys/fs/selinux, /dev/shm, /dev/pts, /proc/sys/fs/binfmt_misc, /dev/mqueue, /dev/hugepages, /sys/fs/fuse/connections, /sys/firmware/efi/efivars. systemd ensures they are mounted even if they are not specified in /etc/fstab or a mount unit.

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