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Re: [PATCH for-4.2 0/4] qcow2: Fix data corruption on XFS

From: Max Reitz
Subject: Re: [PATCH for-4.2 0/4] qcow2: Fix data corruption on XFS
Date: Fri, 1 Nov 2019 13:34:05 +0100
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On 01.11.19 12:20, Max Reitz wrote:
> On 01.11.19 12:16, Vladimir Sementsov-Ogievskiy wrote:
>> 01.11.2019 14:12, Max Reitz wrote:
>>> On 01.11.19 11:28, Vladimir Sementsov-Ogievskiy wrote:
>>>> 01.11.2019 13:20, Max Reitz wrote:
>>>>> On 01.11.19 11:00, Max Reitz wrote:
>>>>>> Hi,
>>>>>> This series builds on the previous RFC.  The workaround is now applied
>>>>>> unconditionally of AIO mode and filesystem because we don’t know those
>>>>>> things for remote filesystems.  Furthermore, bdrv_co_get_self_request()
>>>>>> has been moved to block/io.c.
>>>>>> Applying the workaround unconditionally is fine from a performance
>>>>>> standpoint, because it should actually be dead code, thanks to patch 1
>>>>>> (the elephant in the room).  As far as I know, there is no other block
>>>>>> driver but qcow2 in handle_alloc_space() that would submit zero writes
>>>>>> as part of normal I/O so it can occur concurrently to other write
>>>>>> requests.  It still makes sense to take the workaround for file-posix
>>>>>> because we can’t really prevent that any other block driver will submit
>>>>>> zero writes as part of normal I/O in the future.
>>>>>> Anyway, let’s get to the elephant.
>>>>>>   From input by XFS developers
>>>>>> (https://bugzilla.redhat.com/show_bug.cgi?id=1765547#c7) it seems clear
>>>>>> that c8bb23cbdbe causes fundamental performance problems on XFS with
>>>>>> aio=native that cannot be fixed.  In other cases, c8bb23cbdbe improves
>>>>>> performance or we wouldn’t have it.
>>>>>> In general, avoiding performance regressions is more important than
>>>>>> improving performance, unless the regressions are just a minor corner
>>>>>> case or insignificant when compared to the improvement.  The XFS
>>>>>> regression is no minor corner case, and it isn’t insignificant.  Laurent
>>>>>> Vivier has found performance to decrease by as much as 88 % (on ppc64le,
>>>>>> fio in a guest with 4k blocks, iodepth=8: 1662 kB/s from 13.9 MB/s).
>>>>> Ah, crap.
>>>>> I wanted to send this series as early today as possible to get as much
>>>>> feedback as possible, so I’ve only started doing benchmarks now.
>>>>> The obvious
>>>>> $ qemu-img bench -t none -n -w -S 65536 test.qcow2
>>>>> on XFS takes like 6 seconds on master, and like 50 to 80 seconds with
>>>>> c8bb23cbdbe reverted.  So now on to guest tests...
>>>> Aha, that's very interesting) What about aio-native which should be slowed 
>>>> down?
>>>> Could it be tested like this?
>>> That is aio=native (-n).
>>> But so far I don’t see any significant difference in guest tests (i.e.,
>>> fio --rw=write --bs=4k --iodepth=8 --runtime=1m --direct=1
>>> --ioengine=libaio --thread --numjobs=16 --size=2G --time_based), neither
>>> with 64 kB nor with 2 MB clusters.  (But only on XFS, I’ll have to see
>>> about ext4 still.)
>> hmm, this possibly mostly tests writes to already allocated clusters. Has fio
>> an option to behave like qemu-img bench with -S 65536, i.e. write once into
>> each cluster?
> Maybe, but is that a realistic depiction of whether this change is worth
> it?  That is why I’m doing the guest test, to see whether it actually
> has much impact on the guest.

I’ve changed the above fio invocation to use --rw=randwrite and added
--fallocate=none.  The performance went down, but it went down both with
and without c8bb23cbdbe.

So on my XFS system (XFS on luks on SSD), I see:
- with c8bb23cbdbe: 26.0 - 27.9 MB/s
- without c8bb23cbdbe: 25.6 - 27 MB/s

On my ext4 system (native on SSD), I see:
- with: 39.4 - 41.5 MB/s
- without: 39.4 - 42.0 MB/s

So basically no difference for XFS, and really no difference for ext4.
(I ran these tests with 2 MB clusters.)


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