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

[Max Reitz <mreitz@redhat.com>]

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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”.

Max


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