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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