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

[Peter Lieven]

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?


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.


Peter