Re: [PATCH 3/4] block/mirror: support unaligned write in active mirror

[Vladimir Sementsov-Ogievskiy]

04.10.2019 15:59, Max Reitz wrote:
> On 03.10.19 11:34, Vladimir Sementsov-Ogievskiy wrote:
>> 02.10.2019 18:52, Max Reitz wrote:
>>> On 02.10.19 17:06, Vladimir Sementsov-Ogievskiy wrote:
>>>> 02.10.2019 18:03, Vladimir Sementsov-Ogievskiy wrote:
>>>>> 02.10.2019 17:57, Max Reitz wrote:
>>>>>> On 12.09.19 17:13, Vladimir Sementsov-Ogievskiy wrote:
>>>>>>> Prior 9adc1cb49af8d do_sync_target_write had a bug: it reset aligned-up
>>>>>>> region in the dirty bitmap, which means that we may not copy some bytes
>>>>>>> and assume them copied, which actually leads to producing corrupted
>>>>>>> target.
>>>>>>>
>>>>>>> So 9adc1cb49af8d forced dirty bitmap granularity to be
>>>>>>> request_alignment for mirror-top filter, so we are not working with
>>>>>>> unaligned requests. However forcing large alignment obviously decreases
>>>>>>> performance of unaligned requests.
>>>>>>>
>>>>>>> This commit provides another solution for the problem: if unaligned
>>>>>>> padding is already dirty, we can safely ignore it, as
>>>>>>> 1. It's dirty, it will be copied by mirror_iteration anyway
>>>>>>> 2. It's dirty, so skipping it now we don't increase dirtiness of the
>>>>>>>       bitmap and therefore don't damage "synchronicity" of the
>>>>>>>       write-blocking mirror.
>>>>>>
>>>>>> But that’s not what active mirror is for.  The point of active mirror is
>>>>>> that it must converge because every guest write will contribute towards
>>>>>> that goal.
>>>>>>
>>>>>> If you skip active mirroring for unaligned guest writes, they will not
>>>>>> contribute towards converging, but in fact lead to the opposite.
>>>>>>
>>>>>
>>>>> The will not contribute only if region is already dirty. Actually, after
>>>>> first iteration of mirroring (copying the whole disk), all following 
>>>>> writes
>>>>> will contribute, so the whole process must converge. It is a bit similar 
>>>>> with
>>>>> running one mirror loop in normal mode, and then enable write-blocking.
>>>>>
>>>>
>>>>
>>>> In other words, we don't need "all guest writes contribute" to converge,
>>>> "all guest writes don't create new dirty bits" is enough, as we have 
>>>> parallel
>>>> mirror iteration which contiguously handles dirty bits.
>>>
>>> Hm, in a sense.  But it does mean that guest writes will not contribute
>>> to convergence.
>>>
>>> And that’s against the current definition of write-blocking, which does
>>> state that “when data is written to the source, write it (synchronously)
>>> to the target as well”.
>>>
>>
>> Hmm, understand. But IMHO our proposed behavior is better in general.
>> Do you think it's a problem to change spec now?
>> If yes, I'll resend with an option
> 
> Well, the thing is that I’d find it weird if write-blocking wasn’t
> blocking in all cases.  And in my opinion, it makes more sense for
> active mirror if all writes actively contributed to convergence.
> 

Why? What is the benefit in it?
What is "all writes actively contributed to convergence" for user?

I think for user there may be the following criteria:

1. guaranteed converge, with any guest write load.
Both current and my proposed variants are OK.

2. Less impact on guest.
Obviously my proposed variant is better

3. Total time of mirroring
Seems, current may be a bit better, but I don't think that unaligned
tails gives significant impact.

===

So, assume I want [1]+[2]. And possibly
2.2: Even less impact on guest: ignore not only unaligned tails if they are
already dirty, but full synchronous mirror operation if area is already dirty.

How should I call this? Should it be separate mode, or option for 
write-blocking?

-- 
Best regards,
Vladimir