On Fri, Oct 25, 2019 at 1:11 PM Max Reitz <mre...@redhat.com> 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
Correctness is more important than performance, so this is my preference as a user. Nir > 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.) > > > Max Reitz (3): > block: Make wait/mark serialising requests public > block/file-posix: Detect XFS with CONFIG_FALLOCATE > block/file-posix: Let post-EOF fallocate serialize > > include/block/block_int.h | 3 +++ > block/file-posix.c | 46 +++++++++++++++++++++++++++++++++++++-- > block/io.c | 24 ++++++++++---------- > 3 files changed, 59 insertions(+), 14 deletions(-) > > -- > 2.21.0 > >