[systemd-devel] ConditionPathExists vs mount unit

[Andrei Borzenkov]

10.08.2020 20:59, Böszörményi Zoltán пишет:
> Hi,
> 
> I have to use the same OS image tarball (created by Yocto)
> on several machines with different specifications.
> 
> Where they differ is the disk size and partitioning. On the smaller
> machine (a Sicom SL20 POS hardware, boots from CF card) the disk size
> is too small to have separate partitions for certain purposes that are
> on the other hand mandatory on the larger system.
> 
> The shipped disks are mass-produced and are pre-formatted with
> the same UUIDs across all devices so they are interchangeable.
> 
> So, I discovered the mount unit type:
> https://www.freedesktop.org/software/systemd/man/systemd.mount.html
> 
> This page says that the usual [Unit] section options are applicable.
> 
> I was hoping that the missing partitions can be skipped using the
> ConditionPathExists= option but it seems it's not the case.
> 
> On mount unit looks like this:
> ====================================================
> $ cat var.mount
> [Unit]
> Description=Variable Data (/var)
> ConditionPathExists=/dev/disk/by-uuid/e8282db7-dd6d-4231-b2b1-49887648480c
> ConditionPathIsSymbolicLink=!/var
> DefaultDependencies=no
> Conflicts=umount.target
> Before=local-fs.target umount.target
> After=swap.target
> 
> [Mount]
> What=/dev/disk/by-uuid/e8282db7-dd6d-4231-b2b1-49887648480c
> Where=/var
> Options=noatime
> 
> [Install]
> WantedBy=local-fs.target
> ====================================================
> 
> This boots properly on the larger system where the extra /var
> partition exists but the smaller system fails to boot.
> 
> systemctl status var.mount says:
> 
> Dependency failed for Variable Data (/var)
> var.mount: Job var.mount/start failed with result 'dependency'
> 
> Is there a way to describe optional mounts via such Conditions* options?
> 

No the way you are doing it. Device dependency is checked before
Conditions* directives are even looked at.

If your concern is only boot time, you should consider generators that
will create correct mount units for currently present hardware.