Am 27.06.2018 um 15:37 schrieb Lennart Poettering:
On Mi, 27.06.18 15:09, Ignaz Forster ([email protected]) wrote:
Am 27.06.2018 um 13:39 schrieb Lennart Poettering:
On Mi, 27.06.18 13:02, Ignaz Forster ([email protected]) 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".
Ah, I see - no, that's not the problem here.
The subvolumes are there because we do *not* want to snapshot them.
It's guess it's best to just ignore the second bullet point - it's a
follow up problem, but it isn't really important for the main point:
Attaching a new subvolume to a snapshot.
Ignaz
--
Ignaz Forster <[email protected]>
Research Engineer
SUSE Linux GmbH, Maxfeldstr. 5, D-90409 Nürnberg
Tel: +49-911-74053-281; https://www.suse.com/
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard,
Graham Norton, HRB 21284 (AG Nürnberg)
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