repart: how to add extra attributes to directories

[aplanas]

Maybe this is not a topic for mkosi, but for systemd-repart. Sorry if this is the case and I am posting in the wrong forum.

I am building images that use subvolumes created by systemd-repart. For each subvolume, a new directory is created. There is one subvolume that is special: /@/var. This should be marched as a non-cow, and this is usually done with chatrr +C.

The documentation advise that this is very time sensitive, and if in btrfs this attribute is set to a subvolume that is already populated, there is no guarantee how this content will behave. So this attribute needs to be set just when the subvolume is created and is still empty.

Reading the documentation for systemd-repart seems that those extra attributes should be applied with systemd-tmpfiles, using the h type. IIUC this should be done after the image is created.

The problem is that the buildroot generated by mkosi is already populating some content for /var, so when systemd-repart runs and see the CopyFiles=, it will populate /var before there is a chance to set this +C attribute.

How can this be resolved? Some ideas:

• Make mkosi to support systemd-tmpfiles, and somehow avoid the copyfiles, apply the systemd-tmpfiles rules with --image, and then proceed with the copyfiles.
• Extend systemd-repart to add a new option, like "ExtendedAttributes=" that accept a path and a list of attributes that should be applied, replicating part of systemd-tmpfiles

[DaanDeMeyer]

This can only be solved if mkfs.btrfs from btrfs-progs learns to optionally set nocow on subvolumes it creates. We can then extend systemd-repart to allow setting nocow for subvolumes created via Subvolumes=. If I understand correctly you have some colleagues that are btrfs developers so maybe you can get them to add this to mkfs.btrfs.

[aplanas]

I can try. Yes.