Am 19.05.21 um 18:48 schrieb Kevin Wolf: > Am 19.05.2021 um 15:24 hat Peter Lieven geschrieben: >> Am 20.04.21 um 18:52 schrieb Vladimir Sementsov-Ogievskiy: >>> 20.04.2021 18:04, Kevin Wolf wrote: >>>> Am 20.04.2021 um 16:31 hat Vladimir Sementsov-Ogievskiy geschrieben: >>>>> 15.04.2021 18:22, Kevin Wolf wrote: >>>>>> In order to avoid RMW cycles, is_allocated_sectors() treats zeroed areas >>>>>> like non-zero data if the end of the checked area isn't aligned. This >>>>>> can improve the efficiency of the conversion and was introduced in >>>>>> commit 8dcd3c9b91a. >>>>>> >>>>>> However, it comes with a correctness problem: qemu-img convert is >>>>>> supposed to sparsify areas that contain only zeros, which it doesn't do >>>>>> any more. It turns out that this even happens when not only the >>>>>> unaligned area is zeroed, but also the blocks before and after it. In >>>>>> the bug report, conversion of a fragmented 10G image containing only >>>>>> zeros resulted in an image consuming 2.82 GiB even though the expected >>>>>> size is only 4 KiB. >>>>>> >>>>>> As a tradeoff between both, let's ignore zeroed sectors only after >>>>>> non-zero data to fix the alignment, but if we're only looking at zeros, >>>>>> keep them as such, even if it may mean additional RMW cycles. >>>>>> >>>>> Hmm.. If I understand correctly, we are going to do unaligned >>>>> write-zero. And that helps. >>>> This can happen (mostly raw images on block devices, I think?), but >>>> usually it just means skipping the write because we know that the target >>>> image is already zeroed. >>>> >>>> What it does mean is that if the next part is data, we'll have an >>>> unaligned data write. >>>> >>>>> Doesn't that mean that alignment is wrongly detected? >>>> The problem is that you can have bdrv_block_status_above() return the >>>> same allocation status multiple times in a row, but *pnum can be >>>> unaligned for the conversion. >>>> >>>> We only look at a single range returned by it when detecting the >>>> alignment, so it could be that we have zero buffers for both 0-11 and >>>> 12-16 and detect two misaligned ranges, when both together are a >>>> perfectly aligned zeroed range. >>>> >>>> In theory we could try to do some lookahead and merge ranges where >>>> possible, which should give us the perfect result, but it would make the >>>> code considerably more complicated. (Whether we want to merge them >>>> doesn't only depend on the block status, but possibly also on the >>>> content of a DATA range.) >>>> >>>> Kevin >>>> >>> Oh, I understand now the problem, thanks for explanation. >>> >>> Hmm, yes that means, that if the whole buf is zero, is_allocated_sectors >>> must not align it down, to be possibly "merged" with next chunk if it is >>> zero too. >>> >>> But it's still good to align zeroes down, if data starts somewhere inside >>> the buf, isn't it? >>> >>> what about something like this: >>> >>> diff --git a/qemu-img.c b/qemu-img.c >>> index babb5573ab..d1704584a0 100644 >>> --- a/qemu-img.c >>> +++ b/qemu-img.c >>> @@ -1167,19 +1167,39 @@ static int is_allocated_sectors(const uint8_t *buf, >>> int n, int *pnum, >>> } >>> } >>> >>> + if (i == n) { >>> + /* >>> + * The whole buf is the same. >>> + * >>> + * if it's data, just return it. It's the old behavior. >>> + * >>> + * if it's zero, just return too. It will work good if target is >>> alredy >>> + * zeroed. And if next chunk is zero too we'll have no RMW and no >>> reason >>> + * to write data. >>> + */ >>> + *pnum = i; >>> + return !is_zero; >>> + } >>> + >>> tail = (sector_num + i) & (alignment - 1); >>> if (tail) { >>> if (is_zero && i <= tail) { >>> - /* treat unallocated areas which only consist >>> - * of a small tail as allocated. */ >>> + /* >>> + * For sure next sector after i is data, and it will rewrite >>> this >>> + * tail anyway due to RMW. So, let's just write data now. >>> + */ >>> is_zero = false; >>> } >>> if (!is_zero) { >>> - /* align up end offset of allocated areas. */ >>> + /* If possible, align up end offset of allocated areas. */ >>> i += alignment - tail; >>> i = MIN(i, n); >>> } else { >>> - /* align down end offset of zero areas. */ >>> + /* >>> + * For sure next sector after i is data, and it will rewrite >>> this >>> + * tail anyway due to RMW. Better is avoid RMW and write >>> zeroes up >>> + * to aligned bound. >>> + */ >>> i -= tail; >>> } >>> } >> I think we forgot to follow up on this. Has anyone tested this >> suggestion? >> >> Otherwise, I would try to rerun the tests I did with the my old and >> Kevins suggestion. > I noticed earlier this week that these patches are still in my > development branch, but didn't actually pick it up again yet. So feel > free to try it out.
It seems this time I forgot to follow up. Is this topic still open? Best, Peter