Re: [PATCH] block/rbd: fix write zeroes with growing images

[Peter Lieven]

> Am 18.03.2022 um 17:47 schrieb Stefano Garzarella <sgarzare@redhat.com>:
> 
> On Fri, Mar 18, 2022 at 04:48:18PM +0100, Peter Lieven wrote:
>> 
>> 
>>>> Am 18.03.2022 um 09:25 schrieb Stefano Garzarella <sgarzare@redhat.com>:
>>> 
>>> On Thu, Mar 17, 2022 at 07:27:05PM +0100, Peter Lieven wrote:
>>>> 
>>>> 
>>>>>> Am 17.03.2022 um 17:26 schrieb Stefano Garzarella <sgarzare@redhat.com>:
>>>>> 
>>>>> Commit d24f80234b ("block/rbd: increase dynamically the image size")
>>>>> added a workaround to support growing images (eg. qcow2), resizing
>>>>> the image before write operations that exceed the current size.
>>>>> 
>>>>> We recently added support for write zeroes and without the
>>>>> workaround we can have problems with qcow2.
>>>>> 
>>>>> So let's move the resize into qemu_rbd_start_co() and do it when
>>>>> the command is RBD_AIO_WRITE or RBD_AIO_WRITE_ZEROES.
>>>>> 
>>>>> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2020993
>>>>> Fixes: c56ac27d2a ("block/rbd: add write zeroes support")
>>>>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>>>>> ---
>>>>> block/rbd.c | 26 ++++++++++++++------------
>>>>> 1 file changed, 14 insertions(+), 12 deletions(-)
>>>>> 
>>>>> diff --git a/block/rbd.c b/block/rbd.c
>>>>> index 8f183eba2a..6caf35cbba 100644
>>>>> --- a/block/rbd.c
>>>>> +++ b/block/rbd.c
>>>>> @@ -1107,6 +1107,20 @@ static int coroutine_fn 
>>>>> qemu_rbd_start_co(BlockDriverState *bs,
>>>>> 
>>>>>   assert(!qiov || qiov->size == bytes);
>>>>> 
>>>>> +    if (cmd == RBD_AIO_WRITE || cmd == RBD_AIO_WRITE_ZEROES) {
>>>>> +        /*
>>>>> +         * RBD APIs don't allow us to write more than actual size, so in 
>>>>> order
>>>>> +         * to support growing images, we resize the image before write
>>>>> +         * operations that exceed the current size.
>>>>> +         */
>>>>> +        if (offset + bytes > s->image_size) {
>>>>> +            int r = qemu_rbd_resize(bs, offset + bytes);
>>>>> +            if (r < 0) {
>>>>> +                return r;
>>>>> +            }
>>>>> +        }
>>>>> +    }
>>>>> +
>>>>>   r = rbd_aio_create_completion(&task,
>>>>>                                 (rbd_callback_t) qemu_rbd_completion_cb, 
>>>>> &c);
>>>>>   if (r < 0) {
>>>>> @@ -1182,18 +1196,6 @@ coroutine_fn qemu_rbd_co_pwritev(BlockDriverState 
>>>>> *bs, int64_t offset,
>>>>>                                int64_t bytes, QEMUIOVector *qiov,
>>>>>                                BdrvRequestFlags flags)
>>>>> {
>>>>> -    BDRVRBDState *s = bs->opaque;
>>>>> -    /*
>>>>> -     * RBD APIs don't allow us to write more than actual size, so in 
>>>>> order
>>>>> -     * to support growing images, we resize the image before write
>>>>> -     * operations that exceed the current size.
>>>>> -     */
>>>>> -    if (offset + bytes > s->image_size) {
>>>>> -        int r = qemu_rbd_resize(bs, offset + bytes);
>>>>> -        if (r < 0) {
>>>>> -            return r;
>>>>> -        }
>>>>> -    }
>>>>>   return qemu_rbd_start_co(bs, offset, bytes, qiov, flags, RBD_AIO_WRITE);
>>>>> }
>>>>> 
>>>>> --
>>>>> 2.35.1
>>>>> 
>>>> 
>>>> Do we really have a use case for growing rbd images?
>>> 
>>> The use case is to have a qcow2 image on rbd.
>>> I don't think it's very common, but some people use it and here [1] we had 
>>> a little discussion about features that could be interesting (e.g.  
>>> persistent dirty bitmaps for incremental backup).
>>> 
>>> In any case the support is quite simple and does not affect other use cases 
>>> since we only increase the size when we go beyond the current size.
>>> 
>>> IMHO we can have it in :-)
>>> 
>> 
>> The QCOW2 alone doesn’t make much sense, but additional metadata might be a 
>> use case.
> 
> Yep.
> 
>> Be aware that the current approach will serialize requests. If there is a 
>> real use case, we might think of a better solution.
> 
> Good point, but it only happens when we have to resize, so maybe it's okay 
> for now, but I agree we could do better ;-)

There might also be a problem if a write for a higher offset past eof will be 
executed shortly before a write to a slightly lower offset past eof. The second 
resize will fail as it would shrink the image. We would need proper locking to 
avoid this. Maybe we need to check if we write past eof. If yes, take a lock 
around the resize op and then check again if it’s still eof and only resize if 
true.

Peter

> 
> Thanks,
> Stefano
>