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

[Colin Guthrie]

Hi,

Just wondering if anyone has the latest advice for a future proof setup 
here on Fedora 35 regarding recommendations of base FS etc.

Happy to roll custom systemd installs on top to get latest features etc, 
but don't want to create an inefficient system generally.

Cheers

Col

Colin Guthrie wrote on 06/01/2022 14:42:
> 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
> 
> 
> 


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Colin Guthrie
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