Am 25.10.2019 um 16:19 hat Max Reitz geschrieben: > On 25.10.19 15:56, Vladimir Sementsov-Ogievskiy wrote: > > 25.10.2019 16:40, Vladimir Sementsov-Ogievskiy wrote: > >> 25.10.2019 12:58, Max Reitz wrote: > >>> Hi, > >>> > >>> It seems to me that there is a bug in Linux’s XFS kernel driver, as > >>> I’ve explained here: > >>> > >>> https://lists.nongnu.org/archive/html/qemu-block/2019-10/msg01429.html > >>> > >>> In combination with our commit c8bb23cbdbe32f, this may lead to guest > >>> data corruption when using qcow2 images on XFS with aio=native. > >>> > >>> We can’t wait until the XFS kernel driver is fixed, we should work > >>> around the problem ourselves. > >>> > >>> This is an RFC for two reasons: > >>> (1) I don’t know whether this is the right way to address the issue, > >>> (2) Ideally, we should detect whether the XFS kernel driver is fixed and > >>> if so stop applying the workaround. > >>> I don’t know how we would go about this, so this series doesn’t do > >>> it. (Hence it’s an RFC.) > >>> (3) Perhaps it’s a bit of a layering violation to let the file-posix > >>> driver access and modify a BdrvTrackedRequest object. > >>> > >>> As for how we can address the issue, I see three ways: > >>> (1) The one presented in this series: On XFS with aio=native, we extend > >>> tracked requests for post-EOF fallocate() calls (i.e., write-zero > >>> operations) to reach until infinity (INT64_MAX in practice), mark > >>> them serializing and wait for other conflicting requests. > >>> > >>> Advantages: > >>> + Limits the impact to very specific cases > >>> (And that means it wouldn’t hurt too much to keep this workaround > >>> even when the XFS driver has been fixed) > >>> + Works around the bug where it happens, namely in file-posix > >>> > >>> Disadvantages: > >>> - A bit complex > >>> - A bit of a layering violation (should file-posix have access to > >>> tracked requests?) > >>> > >>> (2) Always skip qcow2’s handle_alloc_space() on XFS. The XFS bug only > >>> becomes visible due to that function: I don’t think qcow2 writes > >>> zeroes in any other I/O path, and raw images are fixed in size so > >>> post-EOF writes won’t happen. > >>> > >>> Advantages: > >>> + Maybe simpler, depending on how difficult it is to handle the > >>> layering violation > >>> + Also fixes the performance problem of handle_alloc_space() being > >>> slow on ppc64+XFS. > >>> > >>> Disadvantages: > >>> - Huge layering violation because qcow2 would need to know whether > >>> the image is stored on XFS or not. > >>> - We’d definitely want to skip this workaround when the XFS driver > >>> has been fixed, so we need some method to find out whether it has > >>> > >>> (3) Drop handle_alloc_space(), i.e. revert c8bb23cbdbe32f. > >>> To my knowledge I’m the only one who has provided any benchmarks for > >>> this commit, and even then I was a bit skeptical because it performs > >>> well in some cases and bad in others. I concluded that it’s > >>> probably worth it because the “some cases” are more likely to occur. > >>> > >>> Now we have this problem of corruption here (granted due to a bug in > >>> the XFS driver), and another report of massively degraded > >>> performance on ppc64 > >>> (https://bugzilla.redhat.com/show_bug.cgi?id=1745823 – sorry, a > >>> private BZ; I hate that :-/ The report is about 40 % worse > >>> performance for an in-guest fio write benchmark.) > >>> > >>> So I have to ask the question about what the justification for > >>> keeping c8bb23cbdbe32f is. How much does performance increase with > >>> it actually? (On non-(ppc64+XFS) machines, obviously) > >>> > >>> Advantages: > >>> + Trivial > >>> + No layering violations > >>> + We wouldn’t need to keep track of whether the kernel bug has been > >>> fixed or not > >>> + Fixes the ppc64+XFS performance problem > >>> > >>> Disadvantages: > >>> - Reverts cluster allocation performance to pre-c8bb23cbdbe32f > >>> levels, whatever that means > >>> > >>> So this is the main reason this is an RFC: What should we do? Is (1) > >>> really the best choice? > >>> > >>> > >>> In any case, I’ve ran the test case I showed in > >>> https://lists.nongnu.org/archive/html/qemu-block/2019-10/msg01282.html > >>> more than ten times with this series applied and the installation > >>> succeeded every time. (Without this series, it fails like every other > >>> time.) > >>> > >>> > >> > >> Hi! > >> > >> First, great thanks for your investigation! > >> > >> We need c8bb23cbdbe3 patch, because we use 1M clusters, and zeroing 1M is > >> significant > >> in time. > >> > >> I've tested a bit: > >> > >> test: > >> for img in /ssd/test.img /test.img; do for cl in 64K 1M; do for step in 4K > >> 64K 1M; do ./qemu-img create -f qcow2 -o cluster_size=$cl $img 15G > > >> /dev/null; printf '%-15s%-7s%-10s : ' $img cl=$cl step=$step; ./qemu-img > >> bench -c $((15 * 1024)) -n -s 4K -S $step -t none -w $img | tail -1 | awk > >> '{print $4}'; done; done; done > >> > >> on master: > >> > >> /ssd/test.img cl=64K step=4K : 0.291 > >> /ssd/test.img cl=64K step=64K : 0.813 > >> /ssd/test.img cl=64K step=1M : 2.799 > >> /ssd/test.img cl=1M step=4K : 0.217 > >> /ssd/test.img cl=1M step=64K : 0.332 > >> /ssd/test.img cl=1M step=1M : 0.685 > >> /test.img cl=64K step=4K : 1.751 > >> /test.img cl=64K step=64K : 14.811 > >> /test.img cl=64K step=1M : 18.321 > >> /test.img cl=1M step=4K : 0.759 > >> /test.img cl=1M step=64K : 13.574 > >> /test.img cl=1M step=1M : 28.970 > >> > >> rerun on master: > >> > >> /ssd/test.img cl=64K step=4K : 0.295 > >> /ssd/test.img cl=64K step=64K : 0.803 > >> /ssd/test.img cl=64K step=1M : 2.921 > >> /ssd/test.img cl=1M step=4K : 0.233 > >> /ssd/test.img cl=1M step=64K : 0.321 > >> /ssd/test.img cl=1M step=1M : 0.762 > >> /test.img cl=64K step=4K : 1.873 > >> /test.img cl=64K step=64K : 15.621 > >> /test.img cl=64K step=1M : 18.428 > >> /test.img cl=1M step=4K : 0.883 > >> /test.img cl=1M step=64K : 13.484 > >> /test.img cl=1M step=1M : 26.244 > >> > >> > >> on master + revert c8bb23cbdbe32f5c326 > >> > >> /ssd/test.img cl=64K step=4K : 0.395 > >> /ssd/test.img cl=64K step=64K : 4.231 > >> /ssd/test.img cl=64K step=1M : 5.598 > >> /ssd/test.img cl=1M step=4K : 0.352 > >> /ssd/test.img cl=1M step=64K : 2.519 > >> /ssd/test.img cl=1M step=1M : 38.919 > >> /test.img cl=64K step=4K : 1.758 > >> /test.img cl=64K step=64K : 9.838 > >> /test.img cl=64K step=1M : 13.384 > >> /test.img cl=1M step=4K : 1.849 > >> /test.img cl=1M step=64K : 19.405 > >> /test.img cl=1M step=1M : 157.090 > >> > >> rerun: > >> > >> /ssd/test.img cl=64K step=4K : 0.407 > >> /ssd/test.img cl=64K step=64K : 3.325 > >> /ssd/test.img cl=64K step=1M : 5.641 > >> /ssd/test.img cl=1M step=4K : 0.346 > >> /ssd/test.img cl=1M step=64K : 2.583 > >> /ssd/test.img cl=1M step=1M : 39.692 > >> /test.img cl=64K step=4K : 1.727 > >> /test.img cl=64K step=64K : 10.058 > >> /test.img cl=64K step=1M : 13.441 > >> /test.img cl=1M step=4K : 1.926 > >> /test.img cl=1M step=64K : 19.738 > >> /test.img cl=1M step=1M : 158.268 > >> > >> > >> So, it's obvious that c8bb23cbdbe32f5c326 is significant for 1M > >> cluster-size, even on rotational > >> disk, which means that previous assumption about calling > >> handle_alloc_space() only for ssd is > >> wrong, we need smarter heuristics.. > >> > >> So, I'd prefer (1) or (2). > > OK. I wonder whether that problem would go away with Berto’s subcluster > series, though. > > > About degradation in some cases: I think the problem is that one (a bit > > larger) > > write may be faster than fast-write-zeroes + small write, as the latter > > means > > additional write to metadata. And it's expected for small clusters in > > conjunction with rotational disk. But the actual limit is dependent on > > specific > > disk. So, I think possible solution is just sometimes try work with > > handle_alloc_space and sometimes without, remember time and length of > > request > > and make dynamic limit... > > Maybe make a decision based both on the ratio of data size to COW area > length (only invoke handle_alloc_space() under a certain threshold), and > the absolute COW area length (always invoke it above a certain > threshold, unless the ratio doesn’t allow it)?
I'm not sure that I would like this level of complexity in this code path... Kevin
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