Re: [PATCH 7/7] block/rbd: change request alignment to 1 byte

[Jason Dillaman]

On Thu, Jan 14, 2021 at 2:59 PM Peter Lieven <pl@kamp.de> wrote:
>
> 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:
> >> since we implement byte interfaces and librbd supports aio on byte 
> >> granularity we can lift
> >> the 512 byte alignment.
> >>
> >> Signed-off-by: Peter Lieven <pl@kamp.de>
> >> ---
> >>  block/rbd.c | 2 --
> >>  1 file changed, 2 deletions(-)
> >>
> >> diff --git a/block/rbd.c b/block/rbd.c
> >> index 27b4404adf..8673e8f553 100644
> >> --- a/block/rbd.c
> >> +++ b/block/rbd.c
> >> @@ -223,8 +223,6 @@ 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;
> > Just a suggestion, but perhaps improve discard alignment, max discard,
> > optimal alignment (if that's something QEMU handles internally) if not
> > overridden by the user.
>
>
> Qemu supports max_discard and discard_alignment. Is there a call to get these 
> limits
>
> from librbd?
>
>
> What do you mean by optimal_alignment? The object size?

krbd does a good job of initializing defaults [1] where optimal and
discard alignment is 64KiB (can actually be 4KiB now), max IO size for
writes, discards, and write-zeroes is the object size * the stripe
count.

> Peter
>
>
>

[1] https://github.com/torvalds/linux/blob/master/drivers/block/rbd.c#L4981

-- 
Jason