Re: how do I know a subvolume is a snapshot?

[Nikolay Borisov <nborisov@suse.com>]

On 17.07.19 г. 20:39 ч., Andrei Borzenkov wrote:
> 17.07.2019 14:19, Nikolay Borisov пишет:
>>
>> This is really odd... So this indeed seems to be a userspace problem. 
> 
> 
> Of course it is user space problem.
> 
> commit 0a0a03554aaf56a6e7245e74fa7d8b3c53f1c20f
> Author: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
> Date:   Fri Mar 23 17:16:49 2018 +0900
> 
>     btrfs-progs: mkfs: add uuid and otime to ROOT_ITEM of, FS_TREE
> 
>     Currently, the top-level subvolume lacks the UUID. As a result, both
>     non-snapshot subvolume and snapshot of top-level subvolume do not have
>     Parent UUID and cannot be distinguisued. Therefore "fi show" of
>     top-level lists all the subvolumes which lacks the UUID in
>     "Snapshot(s)" filed.  Also, it lacks the otime information.
> 
>     Fix this by adding the UUID and otime at the mkfs time.  As a
>     consequence, snapshots of top-level subvolume now have a Parent UUID and
>     UUID tree will create an entry for top-level subvolume at mount time.
>     This should not cause the problem for current kernel, but user program
>     which relies on the empty Parent UUID may be affected by this change.
> 
> 
> What about the question - is there tool to fix existing filesystem by
> adding this information?

Short answer - no. Long answer - it would be trivial to add something
like that to btrfstune but this will only work for root volumes. Because
for snapshots you'd have to iterate all snapshots for a subvol, ensure
they haven't really changed i.e. are real snapshosts and then populate
their Parent UUID to that of the newly set UUID of their respective parent.

>