[systemd-devel] homed: Issues with LUKS storage on btrfs

[Colin Guthrie]

Hi,

Sebastian Wiesner wrote on 28/12/2021 00:04:
> Hello,
> 
> I've experimented with homectl today, and noticed two issues when
> creating LUKS-lookback-backed home areas on top of a btrfs filesystem:
> 
> 1) homectl resize doesn't work reliably on btrfs: It looks as if on
> btrfs resizing a home area requires more free space on the underlying
> btrfs filesystem than I expected.  I assumed that resizing from X to Y
> only requires Y-X extra free space on the underlying device, but on
> btrfs it seems to require Y free space, i.e. it looks as if homectl
> attempts to allocate the entire home area anew.
> 
> I've found https://github.com/systemd/systemd/issues/19398 which looks
> like the problem, but went nowhere.
> 
> 2) homectl creates loopback files which have COW-enabled.  As far as I
> understand btrfs it's not recommended to enable COW for large files
> which frequently get updated in-place which as far as I see it would
> include the backing loopback files for LUKS home areas.
> 
> Shouldn't homectl explicitly disable COW for new home areas if the
> underlying file system is btrfs?
> 
> I can work around 2) by setting -C on /home/ but I haven't figured out
> a solution for 1)
> 
> Is LUKS on btrfs supported by homectl?  Or should I rather use e.g.
> ext4 as underlying file system for /home/?

I'm migrating to a similar setup with a new laptop so would be 
interested in guidance/recommendations on this too if it's available. 
Reddit/other google results seem a little dated.

Managed to get it working on Fedora 35 install with a little PAM 
fighting and selinux tweaks.

Col