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

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH for-4.2 0/4] qcow2: Fix data corruption on XFS
Date: Fri, 1 Nov 2019 11:16:48 +0000

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?

> (Reverting c8bb23cbdbe makes it like 1 to 2 % faster.)
> Max

Best regards,

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