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

[Jason Dillaman]

On Fri, Jan 15, 2021 at 10:39 AM Peter Lieven <pl@kamp.de> wrote:
>
> Am 15.01.21 um 16:27 schrieb 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.
>
>
> Okay, I will have a look at it. If qemu issues a write, discard, write_zero 
> greater than
>
> obj_size  * stripe count will librbd split it internally or will the request 
> fail?

librbd will handle it as needed. My goal is really just to get the
hints down the guest OS.

> Regarding the alignment it seems that rbd_dev->opts->alloc_size is something 
> that comes from the device
>
> configuration and not from rbd? I don't have that information inside the Qemu 
> RBD driver.

librbd doesn't really have the information either. The 64KiB guess
that krbd uses was a compromise since that was the default OSD
allocation size for HDDs since Luminous. Starting with Pacific that
default is going down to 4KiB.

>
> Peter
>
>


-- 
Jason