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

[Jason Dillaman]

On Tue, Jan 19, 2021 at 4:36 AM Peter Lieven <pl@kamp.de> wrote:
>
> Am 18.01.21 um 23:33 schrieb 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.
>
>
> I will try to adjust these values as far as it is possible and makes sense.
>
>
> Is there a way to check the minimum supported OSD release in the backend from 
> librbd / librados?

It's not a minimum -- RADOS will gladly access 1 byte writes as well.
It's really just the optimal (performance and space-wise). Sadly,
there is no realistic way to query this data from the backend.

>
> Anyway, I want to sent a V2 by the end of this week.
>
>
> Peter
>
>


-- 
Jason