Re: [RFC 0/3] block/file-posix: Work around XFS bug

[Max Reitz <mreitz@redhat.com>]

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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)?

Max


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