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

[Vladimir Sementsov-Ogievskiy]

25.10.2019 17:19, Max Reitz wrote:
> 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.

Very possible, I thought about it too.

> 
>> 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)?
> 

Yes, something like this..

without handle_alloc_space, time = time(write aligned up to cluster)
with handle_alloc_space, time = time(fast zero write) + time(original write)

If we have some statistics on normal-write vs zero-write timing, we can just
calculate both variants and choose faster.

if (predict_zero_write_time(aligned up request) + predict_write_time(request) < 
predict_write_time(aligned up request)) {
    use handle_alloc_space()
}


-- 
Best regards,
Vladimir