empty directory from previous subvolume in a snapshot is not sent|received

empty directory from previous subvolume in a snapshot is not sent|received

[Christoph Anton Mitterer]

Hey.

Not sure whether this is a bug or expected.

On my btrfs I have subvolumes like:
  data/
  2023-06-21/
where e.g. data/ contains the root filesystem and 2023-06-21/ is a ro-
snapshot thereof.


When I created 2023-06-21/ from data/, the latter contained another
(rw-)subvolume data/pictures/, which I've deleted (actually: moved out
the files back to data/ and rmdir-ed the now empty subvol... or maybe I
did btrfs subvolume delete - not sure anymore) again after creating the
snapshot.


Now 2023-06-21/ contains an empty (non-subvolume) 2023-06-21/pictures/,
which is expected.


Today I've send|received 2023-06-21/ to another btrfs (at that point,
the original data/pictures/ subvolume was already gone), and diff -qr -
-no-dereference-ed the two afterwards.

Outcome (apart from "differing" files/sockets/block/char special files)
is that the target doesn't contain the empty pictures/ dir.


Not a big problem for me,... but is this expected or some kind of
strange bug?


Thanks,
Chris.

Re: empty directory from previous subvolume in a snapshot is not sent|received

[Andrei Borzenkov]

On 22.06.2023 16:34, Christoph Anton Mitterer wrote:
> Hey.
> 
> Not sure whether this is a bug or expected.
> 
> On my btrfs I have subvolumes like:
>    data/
>    2023-06-21/
> where e.g. data/ contains the root filesystem and 2023-06-21/ is a ro-
> snapshot thereof.
> 
> 
> When I created 2023-06-21/ from data/, the latter contained another
> (rw-)subvolume data/pictures/, which I've deleted (actually: moved out
> the files back to data/ and rmdir-ed the now empty subvol... or maybe I
> did btrfs subvolume delete - not sure anymore) again after creating the
> snapshot.
> 
> 
> Now 2023-06-21/ contains an empty (non-subvolume) 2023-06-21/pictures/,
> which is expected.
> 
> 
> Today I've send|received 2023-06-21/ to another btrfs (at that point,
> the original data/pictures/ subvolume was already gone), and diff -qr -
> -no-dereference-ed the two afterwards.
> 
> Outcome (apart from "differing" files/sockets/block/char special files)
> is that the target doesn't contain the empty pictures/ dir.
> 
> 
> Not a big problem for me,... but is this expected or some kind of
> strange bug?
> 
> 

I think it is expected. btrfs does not support either recursive 
snapshots or recursive send so "btrfs send" skips directory entry that 
points to subvolume root.

Re: empty directory from previous subvolume in a snapshot is not sent|received

[Christoph Anton Mitterer]

On Thu, 2023-06-22 at 18:22 +0300, Andrei Borzenkov wrote:
> I think it is expected. btrfs does not support either recursive 
> snapshots or recursive send so "btrfs send" skips directory entry
> that 
> points to subvolume root.

Well it's clear that the sub-volume is not contained in the snapshot or
sent.

But in the snapshot (on the original fs) I have the empty directory
from the subvolume that was not recursively snapshotted.
Shouldn't that be just a plain empty directory... and thus be
sent|received (as empty directory) when I send|receive the snapshot?


Cheers,
Chris.

Re: empty directory from previous subvolume in a snapshot is not sent|received

[Andrei Borzenkov]

On 22.06.2023 19:09, Christoph Anton Mitterer wrote:
> On Thu, 2023-06-22 at 18:22 +0300, Andrei Borzenkov wrote:
>> I think it is expected. btrfs does not support either recursive
>> snapshots or recursive send so "btrfs send" skips directory entry
>> that
>> points to subvolume root.
> 
> Well it's clear that the sub-volume is not contained in the snapshot or
> sent.
> 
> But in the snapshot (on the original fs) I have the empty directory
> from the subvolume that was not recursively snapshotted.

It is not directory. It is "orphan subvolume graft point" for the lack 
of better term.

tw:/mnt # btrfs subvolume create ./data/pictures
Create subvolume './data/pictures'
tw:/mnt # btrfs subvolume snapshot data/ snap1
Create a snapshot of 'data/' in './snap1'
tw:/mnt # ll snap1
total 0
drwxr-xr-x 1 root root 0 Jun 23 07:53 pictures
tw:/mnt # touch snap1/pictures/foo
touch: cannot touch 'snap1/pictures/foo': Permission denied
tw:/mnt #


> Shouldn't that be just a plain empty directory... and thus be
> sent|received (as empty directory) when I send|receive the snapshot?
> 
> 
> Cheers,
> Chris.

Re: empty directory from previous subvolume in a snapshot is not sent|received

[Christoph Anton Mitterer]

Hey.


On Fri, 2023-06-23 at 07:55 +0300, Andrei Borzenkov wrote:
> It is not directory. It is "orphan subvolume graft point" for the
> lack 
> of better term.

Ah I see. Thanks.

I think this deserves being added to the btrfs(5) manpage, so I made a
ticket about it at:
https://github.com/kdave/btrfs-progs/issues/644


Thanks,
Chris.