14.06.2019 19:10, Max Reitz wrote:
> On 14.06.19 17:22, Vladimir Sementsov-Ogievskiy wrote:
>> 13.06.2019 1:09, Max Reitz wrote:
>>> If the top node's driver does not provide snapshot functionality and we
>>> want to go down the chain, we should go towards the child which stores
>>> the data, i.e. the storage child.
>>>
>>> bdrv_snapshot_goto() becomes a bit weird because we may have to redirect
>>> the actual child pointer, so it only works if the storage child is
>>> bs->file or bs->backing (and then we have to find out which it is).
>>>
>>> Signed-off-by: Max Reitz <mre...@redhat.com>
>>> ---
>>> block/snapshot.c | 74 ++++++++++++++++++++++++++++++++++--------------
>>> 1 file changed, 53 insertions(+), 21 deletions(-)
>>>
>>> diff --git a/block/snapshot.c b/block/snapshot.c
>>> index f2f48f926a..58cd667f3a 100644
>>> --- a/block/snapshot.c
>>> +++ b/block/snapshot.c
>>> @@ -154,8 +154,9 @@ int bdrv_can_snapshot(BlockDriverState *bs)
>>> }
>>>
>>> if (!drv->bdrv_snapshot_create) {
>>> - if (bs->file != NULL) {
>>> - return bdrv_can_snapshot(bs->file->bs);
>>> + BlockDriverState *storage_bs = bdrv_storage_bs(bs);
>>> + if (storage_bs) {
>>> + return bdrv_can_snapshot(storage_bs);
>>> }
>>> return 0;
>>> }
>>
>> Hmm is it correct at all doing a snapshot, when top format node doesn't
>> support it,
>> metadata child doesn't support it and storage child supports? Doing
>> snapshots of
>> storage child seems useless, as data file must be in sync with metadata.
>
> You’re right.
>
> That’s actually a bug already. VMDK can store data in multiple
> children, but it does not support snapshots. So if you store such a
> split VMDK disk on an RBD volume, it is possible that just the
> descriptor file is snapshotted, but nothing else.
>
> Hmmm. I think the best way is to check whether there is exactly one
> child that is not the bdrv_filtered_cow_child(). If so, we can go down
> to it and snapshot it. Otherwise, the node does not support snapshots.
>
Anything prevents format node to store something not in a bdrv child but in
something separate?
May be the safest way is to fall back only for filter nodes.
--
Best regards,
Vladimir
