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

[Peter Lieven]

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?


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


Peter