Re: [PATCH 6/7] block/rbd: add write zeroes support

[Peter Lieven <pl@kamp.de>]

Am 14.01.21 um 20:19 schrieb Jason Dillaman:
> On Sun, Dec 27, 2020 at 11:42 AM Peter Lieven <pl@kamp.de> wrote:
>> Signed-off-by: Peter Lieven <pl@kamp.de>
>> ---
>>  block/rbd.c | 31 ++++++++++++++++++++++++++++++-
>>  1 file changed, 30 insertions(+), 1 deletion(-)
>>
>> diff --git a/block/rbd.c b/block/rbd.c
>> index 2d77d0007f..27b4404adf 100644
>> --- a/block/rbd.c
>> +++ b/block/rbd.c
>> @@ -63,7 +63,8 @@ typedef enum {
>>      RBD_AIO_READ,
>>      RBD_AIO_WRITE,
>>      RBD_AIO_DISCARD,
>> -    RBD_AIO_FLUSH
>> +    RBD_AIO_FLUSH,
>> +    RBD_AIO_WRITE_ZEROES
>>  } RBDAIOCmd;
>>
>>  typedef struct BDRVRBDState {
>> @@ -221,8 +222,12 @@ done:
>>
>>  static void qemu_rbd_refresh_limits(BlockDriverState *bs, Error **errp)
>>  {
>> +    BDRVRBDState *s = bs->opaque;
>>      /* XXX Does RBD support AIO on less than 512-byte alignment? */
>>      bs->bl.request_alignment = 512;
>> +#ifdef LIBRBD_SUPPORTS_WRITE_ZEROES
>> +    bs->bl.pwrite_zeroes_alignment = s->object_size;
> The optimal alignment is 512 bytes -- but it can safely work just fine
> down to 1 byte alignment since it will pad the request with additional
> writes if needed.


Okay and this will likely be faster than having Qemu doing that request split, right?

If we drop the alignment hint Qemu will pass the original request.


>
>> +#endif
>>  }
>>
>>
>> @@ -695,6 +700,9 @@ static int qemu_rbd_open(BlockDriverState *bs, QDict *options, int flags,
>>      }
>>
>>      s->aio_context = bdrv_get_aio_context(bs);
>> +#ifdef LIBRBD_SUPPORTS_WRITE_ZEROES
>> +    bs->supported_zero_flags = BDRV_REQ_MAY_UNMAP;
>> +#endif
>>
>>      /* When extending regular files, we get zeros from the OS */
>>      bs->supported_truncate_flags = BDRV_REQ_ZERO_WRITE;
>> @@ -808,6 +816,11 @@ static int coroutine_fn qemu_rbd_start_co(BlockDriverState *bs,
>>      case RBD_AIO_FLUSH:
>>          r = rbd_aio_flush(s->image, c);
>>          break;
>> +#ifdef LIBRBD_SUPPORTS_WRITE_ZEROES
>> +    case RBD_AIO_WRITE_ZEROES:
>> +        r = rbd_aio_write_zeroes(s->image, offset, bytes, c, 0, 0);
>> +        break;
>> +#endif
>>      default:
>>          r = -EINVAL;
>>      }
>> @@ -880,6 +893,19 @@ static int coroutine_fn qemu_rbd_co_pdiscard(BlockDriverState *bs,
>>      return qemu_rbd_start_co(bs, offset, count, NULL, 0, RBD_AIO_DISCARD);
>>  }
>>
>> +#ifdef LIBRBD_SUPPORTS_WRITE_ZEROES
>> +static int
>> +coroutine_fn qemu_rbd_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset,
>> +                                      int count, BdrvRequestFlags flags)
>> +{
>> +    if (!(flags & BDRV_REQ_MAY_UNMAP)) {
>> +        return -ENOTSUP;
>> +    }
> There is a "RBD_WRITE_ZEROES_FLAG_THICK_PROVISION" flag that you can
> use to optionally disable unmap.


I have seen that. If you want I can support for this, too. But afaik this

is only available since Octopus release?


Peter