Re: find snapshot parent?

[Andrei Borzenkov <arvidjaar@gmail.com>]

07.07.2019 9:43, Nikolay Borisov пишет:
> 
> 
> On 6.07.19 г. 23:43 ч., Ulli Horlacher wrote:
>> On Sat 2019-07-06 (19:57), Nikolay Borisov wrote:
>>
>>>> And how can I see whether /test/tmp/xx/ss1 is a snapshot at all?
>>>> Do all snapshots have a "Parent UUID" and regular subvolumes not?
>>>
>>> Indeed, only snapshots have a Parent UUID.
>>
>> Not all:
>>
>> root@xerus:/test# btrfs subvolume snapshot -r /test /test/ss1
>> Create a readonly snapshot of '/test' in '/test/ss1'
>>
>> root@xerus:/test# btrfs subvolume show /test/ss1
>> /test/ss1
>>         Name:                   ss1
>>         UUID:                   02bd07bc-0bab-3442-96be-40790e1ba9be
>>         Parent UUID:            -
>>         Received UUID:          -
>>         Creation time:          2019-07-06 22:37:37 +0200
>>         Subvolume ID:           1036
>>         Generation:             9824
>>         Gen at creation:        9824
>>         Parent ID:              5
>>         Top level ID:           5
>>         Flags:                  readonly
>>         Snapshot(s):
>>
>> root@xerus:/test# btrfs subvolume show /test
>> /test is toplevel subvolume
> 
> This is really odd, looking at create_pending_snapshot the codes : 
> 
> memcpy(new_root_item->parent_uuid, root->root_item.uuid,                                      
>                         BTRFS_UUID_SIZE);  
> 
> And that's not conditional on whether the snapshot is read only or not. 
> So everytime we creata a snapshot it ought to be receiving the parent's 
> subvolume UUID in its parent_uuid field.

Does top level subvolume of btrfs have subvolume UUID at all? How can
one display it? None of "btrfs subvolume" commands show it.

> And indeed testing with latest misc-next kernel: 
> 
> root@ubuntu-virtual:~# btrfs subvol create /media/scratch/subvol10 
> Create subvolume '/media/scratch/subvol10'
> 
> root@ubuntu-virtual:~# btrfs subvol snapshot /media/scratch/subvol10/ /media/scratch/snap-subvol10
> Create a snapshot of '/media/scratch/subvol10/' in '/media/scratch/snap-subvol10'
> 

/media/scratch/subvol10 is not top level subvolume.