Re: [PATCH V3 5/6] block/rbd: add write zeroes support

[Ilya Dryomov <idryomov@gmail.com>]

On Fri, Jun 18, 2021 at 11:00 AM Peter Lieven <pl@kamp.de> wrote:
>
> Am 16.06.21 um 14:34 schrieb Ilya Dryomov:
> > On Wed, May 19, 2021 at 4:28 PM Peter Lieven <pl@kamp.de> wrote:
> >> Signed-off-by: Peter Lieven <pl@kamp.de>
> >> ---
> >>  block/rbd.c | 37 ++++++++++++++++++++++++++++++++++++-
> >>  1 file changed, 36 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/block/rbd.c b/block/rbd.c
> >> index 0d8612a988..ee13f08a74 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 {
> >> @@ -705,6 +706,10 @@ static int qemu_rbd_open(BlockDriverState *bs, QDict *options, int flags,
> >>          }
> >>      }
> >>
> >> +#ifdef LIBRBD_SUPPORTS_WRITE_ZEROES
> >> +    bs->supported_zero_flags = BDRV_REQ_MAY_UNMAP;
> > I wonder if we should also set BDRV_REQ_NO_FALLBACK here since librbd
> > does not really have a notion of non-efficient explicit zeroing.
>
>
> This is only true if thick provisioning is supported which is in Octopus onwards, right?

Since Pacific, I think.

>
> So it would only be correct to set this if thick provisioning is supported otherwise we could
>
> fail with ENOTSUP and then qemu emulates the zeroing with plain writes.

I actually had a question about that.  Why are you returning ENOTSUP
in case BDRV_REQ_MAY_UNMAP is not specified and that can't be fulfilled
because librbd is too old for RBD_WRITE_ZEROES_FLAG_THICK_PROVISION?

My understanding has always been that BDRV_REQ_MAY_UNMAP is just
a hint.  Deallocating if BDRV_REQ_MAY_UNMAP is specified is not nice
but should be perfectly acceptable.  It is certainly better than
returning ENOTSUP, particularly if ENOTSUP causes Qemu to do plain
zeroing.

Thanks,

                Ilya