systemd-tmpfiles subvolume handling vs. changing default btrfs root

[lennart@xxxxxxxxxxxxxx (Lennart Poettering)]

On Mi, 27.06.18 15:09, Ignaz Forster (iforster at suse.de) wrote:

> Am 27.06.2018 um 13:39 schrieb Lennart Poettering:
> > On Mi, 27.06.18 13:02, Ignaz Forster (iforster at suse.de) wrote:
> > 
> > > Hello,
> > > 
> > > when using systemd-tmpfiles' feature to create subvolumes it will always
> > > create the new subvolume as a child of the subvolume of the given path. This
> > > however may not always be the expected parent, especially when using btrfs
> > > snapshots to switch between various system states.
> > > 
> > > Example layout:
> > > ===============
> > > 
> > > Let's assume the following subvolume layout (a simplified openSUSE layout):
> > > 
> > > ID	parent	top level	path	
> > > --	------	---------	----
> > > 257	5	5		<FS_TREE>/@
> > > 258	257	257		<FS_TREE>/@/var
> > > 259	257	257		<FS_TREE>/@/.snapshots/1/snapshot
> > > 260	257	257		<FS_TREE>/@/.snapshots/2/snapshot
> > > 261	257	257		<FS_TREE>/@/.snapshots/3/snapshot
> > > 
> > > A corresponding /etc/fstab could look like this:
> > > 
> > > /dev/sdx	/	btrfs	defaults	0	0
> > > /dev/sdx	/var	btrfs	subvol=@/var	0	0
> > > 
> > > with the default btrfs subvolume set to "261".
> > > The third snapshot would thus be the root file system, with /var mounted on
> > > top of it.
> > > 
> > > 
> > > The problem:
> > > ============
> > > 
> > > Creating "/var/test" would create a new entry like
> > > 262	258	258		@/var/test
> > > as expected.
> > > However creating "/opt" would create an entry similar to the following:
> > > 263	261	261		@/.snapshots/3/snapshot/opt
> > > 
> > > This is not good, as two things will happen now:
> > > * When changing the snapshot (e.g. by reverting back to an old snapshot or
> > > creating a new one) /opt won't be visible any more (without manually
> > > mounting it), as it is not nested into the existing structure any more
> > > * The third snapshot cannot be deleted without removing the
> > > subvolume first
> > 
> > I am not sure I follow here fully. but isn't this just a shortcoming because
> > you are not doing recursive snapshots? why not just fix that?
> 
> With "recursive snapshots" I assume you mean putting the snapshot below the
> original root file system?

By recursive snaphots I really mean recursive snapshots, i.e. if you
have a subvolume called `/foobar` and there's a subvolume below it
called `/foobar/var`, and you'd make a snapshot of `/foobar` and call
it `/foobar2`, then this would implicitly also have the effect of
snapshotting `/foobar/var` and calling it `/foobar2/var`, so that each
snapshot is always "complete".

Lennart

-- 
Lennart Poettering, Red Hat