systemd: Setup Overlay-Filesystem on Debian 12

Shimoda

2026-07-08 12:00

An Overlay-Filesystem is useful to reduce read/write access to the root-filesystem. Minimizing root filesystem access extends the lifespan of the main SD card or flash memory device in SBC (Single Board Computer) environments.

This article describes how to set it up with systemd and its mount or service units.

Mount the another device

Create a mount point and systemd mount unit.

sudo mkdir -p /mnt/external
sudo systemctl edit --full mnt-external.mount

[Unit]
Description=Extra Device Storage
Documentation=man:fstab(5) man:systemd-fstab-generator(8)

# dependencies...
DefaultDependencies=no
Conflicts=umount.target
After=
Before=local-fs.target umount.target

[Mount]
What=/dev/sda1
Where=/mnt/external
Type=ext4
Options=

[Install]
WantedBy=local-fs.target

Mount Overlay-Filesystem

Create the overlay filesystem directory on the secondary storage and make a overlay-filesystem mount unit.

sudo mkdir -p /mnt/external/var-log{,.tmp}
sudo systemctl edit --full --force var-log.mount

[Unit]
Description=Overlay Mount for Log Directory
Documentation=man:fstab(5) man:systemd-fstab-generator(8)

# dependencies...
DefaultDependencies=no
Conflicts=umount.target
Requires=mnt-external.mount
After=mnt-external.mount
Before=local-fs.target umount.target

[Mount]
What=overlay
Where=/var/log
Type=overlay
Options=index=off,metacopy=off,workdir=/mnt/external/var-log.tmp,lowerdir=/var/log,upperdir=/mnt/external/var-log

[Install]
# mount automatically after booting
WantedBy=local-fs.target

Restart several services after var-log

Create a systemd service unit that depends on the previous mount unit.

sudo systemctl edit --full after-var-log.service

[Unit]
Description=Run Custom Boot Script after var-log mount

DefaultDependencies=no
Conflicts=shutdown.target
Requires=var-log.mount
After=var-log.mount
Before=shutdown.target

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/bin/systemctl restart systemd-journald.service

[Install]
WantedBy=multi-user.target

Enable mount and service units

Finally, enable created mount and service units. These will be activated in this step and will take effect from next boot.

$ sudo systemctl daemon-reload
$ sudo systemctl enable mnt-external.mount
$ sudo systemctl enable var-log.mount
$ sudo systemctl enable after-var-log.service

Mount the another device

Mount Overlay-Filesystem

Restart several services after var-log

Enable mount and service units

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