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. Max
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