Systemd Unit to activate loopback devices before LVM

In a Debian server I'm using LVM to create a single logical volume from multiple different volumes. One of the volumes is a loop-back device which refers to a file in another filesystem.

The loop-back device needs to be activated before the LVM service starts or the later will fail due to missing volumes. To do so a special systemd unit needs to be created which will not have the default dependencies of units and will get executed before lvm2-activation-early service.

Systemd will set a number of dependencies for all units by default to bring the system into a usable state before starting most of the units. This behavior is controlled by DefaultDependencies flag. Leaving DefaultDependencies to its default True value creates a dependency loop which systemd will forcefully break to finish booting the system. Obviously this non-deterministic flow can result in different than desired execution order which in turn will fail the LVM volume activation.

Setting DefaultDependencies to False will disable all but essential dependencies and will allow our unit to execute in time. Systemd manual confirms that we can set the option to false:

Generally, only services involved with early boot or late shutdown should set this option to false.

The second is to execute before lvm2-activation-early. This is simply achieved by setting Before=lvm2-activation-early.

The third and last step is to set the command to execute. In my case it's /sbin/losetup /dev/loop0 /volume.img as I want to create /dev/loop0 from the file /volume.img. Set the process type to oneshot so systemd waits for the process to exit before it starts follow-up units. Again from the systemd manual

Behavior of oneshot is similar to simple; however, it is expected that the process has to exit before systemd starts follow-up units.

Place the unit file in /etc/systemd/system and in the next reboot the loop-back device should be available to LVM.

Here's the final unit file:

Description=Activate loop device

ExecStart=/sbin/losetup /dev/loop0 /volume.img


See also: - Anthony's excellent LVM Loopback How-To

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