03.08.2019 0:19, Max Reitz wrote: > On 02.08.19 20:58, Vladimir Sementsov-Ogievskiy wrote: >> hbitmap_reset is broken: it rounds up the requested region. It leads to >> the following bug, which is shown by fixed test: >> >> assume granularity = 2 >> set(0, 3) # count becomes 4 >> reset(0, 1) # count becomes 2 >> >> But user of the interface assume that virtual bit 1 should be still >> dirty, so hbitmap should report count to be 4! >> >> In other words, because of granularity, when we set one "virtual" bit, >> yes, we make all "virtual" bits in same chunk to be dirty. But this >> should not be so for reset. >> >> Fix this, aligning bound correctly. >> >> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com> >> --- >> >> Hi all! >> >> Hmm, is it a bug or feature? :) >> I don't have a test for mirror yet, but I think that sync mirror may be >> broken >> because of this, as do_sync_target_write() seems to be using unaligned reset. > > Crap. > > > Yes, you’re right. This would fix it, and it wouldn’t fix it in the > worst way. > > But I don’t know whether this patch is the best way forward still. I > think call hbitmap_reset() with unaligned boundaries generally calls for > trouble, as John has laid out. If mirror’s do_sync_target_write() is > the only offender right now, I’d prefer for hbitmap_reset() to assert > that the boundaries are aligned (for 4.2),
OK, agree that asserting this is better. and for > do_sync_target_write() to be fixed (for 4.1? :-/). > > (A practical problem with this patch is that do_sync_target_write() will > still do the write, but it won’t change anything in the bitmap, so the > copy operation was effectively useless.) > > I don’t know how to fix mirror exactly, though. I have four ideas: > > (A) Quick fix 1: do_sync_target_write() should shrink [offset, offset + > bytes) such that it is aligned. This would make it skip writes that > don’t fill one whole chunk. > > +: Simple fix. Could go into 4.1. > -: Makes copy-mode=write-blocking equal to copy-mode=background unless > you set the granularity to like 512. (Still beats just being > completely broken.) > > (B) Quick fix 2: Setting the request_alignment block limit to the job’s > granularity when in write-blocking mode. > > +: Very simple fix. Could go into 4.1. > +: Every write will trigger a RMW cycle, which copies the whole chunk to > the target, so write-blocking will do what it’s supposed to do. > -: request_alignment forces everything to have the same granularity, so > this slows down reads needlessly. (But only for write-blocking.) > > (C) Maybe the right fix 1: Let do_sync_target_write() expand [offset, > offset + bytes) such that it is aligned and read head and tail from the > source node. (So it would do the RMW itself.) > > + Doesn’t slow reads down. > + Writes to dirty areas will make them clean – which is what > write-blocking is for. > - Probably more complicated. Nothing for 4.1. This is how backup works. > > (D) Maybe the right fix 2: Split BlockLimits.request_alignment into > read_alignment and write_alignment. Then do (B). Now it's OK, but if we implement bitmap mode for mirror (which is upcoming anyway, I think), it will slow down all writes, when we are interested only in which are touching dirty parts. > > In effect, this is more or less the same as (C), but probably in a > simpler way. Still not simple enough for 4.1, though. > > > So... I tend to do either (A) or (B) now, and then probably (D) for > 4.2? (And because (D) is an extension to (B), it would make sense to do > (B) now, unless you’d prefer (A).) > > Max > -- Best regards, Vladimir
