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Re: [RFC 0/3] block/file-posix: Work around XFS bug
From: |
Vladimir Sementsov-Ogievskiy |
Subject: |
Re: [RFC 0/3] block/file-posix: Work around XFS bug |
Date: |
Sat, 26 Oct 2019 17:52:45 +0000 |
26.10.2019 20:37, Nir Soffer wrote:
> On Fri, Oct 25, 2019 at 1:11 PM Max Reitz <address@hidden> 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.
>
Hmm, still, incorrect is XFS, not Qemu. This bug may be triggered by another
software, or may be another scenario in Qemu (not sure).
>
>> 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
>>
>>
--
Best regards,
Vladimir
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, (continued)
Re: [RFC 0/3] block/file-posix: Work around XFS bug, Peter Maydell, 2019/10/25
Re: [RFC 0/3] block/file-posix: Work around XFS bug, no-reply, 2019/10/25
Re: [RFC 0/3] block/file-posix: Work around XFS bug, Nir Soffer, 2019/10/26
- Re: [RFC 0/3] block/file-posix: Work around XFS bug,
Vladimir Sementsov-Ogievskiy <=
Re: [RFC 0/3] block/file-posix: Work around XFS bug, Stefan Hajnoczi, 2019/10/27
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Vladimir Sementsov-Ogievskiy, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Vladimir Sementsov-Ogievskiy, 2019/10/28
Re: [RFC 0/3] block/file-posix: Work around XFS bug, Kevin Wolf, 2019/10/28