Re: [PATCH V2] Btrfs: keep dropped roots in cache until transaction commit

[Josef Bacik <jbacik@fb.com>]

On 09/15/2015 11:50 AM, Holger Hoffstätte wrote:
> On 09/15/15 16:07, Josef Bacik wrote:
>> When dropping a snapshot we need to account for the qgroup changes.  If we drop
>> the snapshot in all one go then the backref code will fail to find blocks from
>> the snapshot we dropped since it won't be able to find the root in the fs root
>> cache.  This can lead to us failing to find refs from other roots that pointed
>> at blocks in the now deleted root.  To handle this we need to not remove the fs
>> roots from the cache until after we process the qgroup operations.  Do this by
>> adding dropped roots to a list on the transaction, and letting the transaction
>> remove the roots at the same time it drops the commit roots.  This will keep all
>> of the backref searching code in sync properly, and fixes a problem Mark was
>> seeing with snapshot delete and qgroups.  Thanks,
>>
>> Signed-off-by: Josef Bacik <jbacik@fb.com>
>> ---
>> V1->V2:
>> -clear the trans tag when we are finished dropping the subvol so we don't try to
>>    update the root during commit.
>
> This V2 does indeed seem to fix the issues I reported with snapshot deletion &
> concurrent sync. I've now created/filled/deleted countless snapshots while issuing
> sync(s) in parallel, and the problem that I saw fairly frequently with V1 no longer
> seems to occur here. Therefore:
>
> Tested-by: Holger Hoffstätte <holger.hoffstaette@googlemail.com>
>
> Thanks for the quick fix, Josef!
>

Great thanks,

Josef