[systemd-devel] the need for a discoverable sub-volumes specification

[Lennart Poettering]

On Do, 18.11.21 14:51, Chris Murphy (lists at colorremedies.com) wrote:

> How to do swapfiles?

Is this really a concept that deserves too much attention? I mean, I
have the suspicion that half the benefit of swap space is that it can
act as backing store for hibernation. But swap files are icky for that
since that means the resume code has to mount the fs first, but given
the fs is dirty during the hibernation state this is highly problematic.

Hence, I have the suspicion that if you do swap you should probably do
swap partitions, not swap files, because it can cover all usecase:
paging *and* hibernation.

> Currently I'm creating a "swap" subvolume in the top-level of the file
> system and /etc/fstab looks like this
>
> UUID=$FSUUID    /var/swap               btrfs   noatime,subvol=swap 0 0
> /var/swap/swapfile1 none swap defaults 0 0
>
> This seems to work reliably after hundreds of boots.
>
> a. Is this naming convention for the subvolume adequate? Seems like it
> can just be "swap" because the GPT method is just a single partition
> type GUID that's shared by multiboot Linux setups, i.e. not arch or
> distro specific

I'd still put it one level down, and marke it with some non-typical
character so that it is less likely to clash with anything else.

> b. Is the mount point, /var/swap, OK?

I see no reason why not.

> c. What should the additional naming convention be for the swapfile
> itself so swapon happens automatically?

To me it appears these things should be distinct: if automatic
activation of swap files is desirable, then there should probably be a
systemd generator that finds all suitable files in /var/swap/ and
generates .swap units for them. This would then work with any kind of
setup, i.e. independently of the btrfs auto-discovery stuff. The other
thing would be the btrfs auto-disocvery to then actually mount
something there automatically.

> Also, instead of /@auto/ I'm wondering if we could have
> /x-systemd.auto/ ? This makes it more clearly systemd's namespace, and
> while I'm a big fan of the @ symbol for typographic history reasons,
> it's being used in the subvolume/snapshot regimes rather haphazardly
> for different purposes which might be confusing? e.g. Timeshift
> expects subvolumes it manages to be prefixed with @. Meanwhile SUSE
> uses @ for its (visible) root subvolume in which everything else goes.
> And still ZFS uses @ for their (read-only) snapshots.

I try to keep the "systemd" name out of entirely generic specs, since
there are some people who have an issue with that. i.e. this way we
tricked even Devuan to adopt /etc/os-release and the /run/ hierarchy,
since they probably aren't even aware that these are systemd things.

Other chars could be used too: /+auto/ sounds OK to me too. or
/_auto/, or /=auto/ or so.

Lennart

--
Lennart Poettering, Berlin