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.) 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
