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From: "Denis V. Lunev" <den@openvz.org>
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Date: Thu, 2 Jun 2016 21:34:08 +0300
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Subject: Re: [Qemu-devel] [PATCH v2 1/1] qcow2: avoid extra flushes in qcow2
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To: qemu-block@nongnu.org, qemu-devel@nongnu.org
Cc: Pavel Borzenkov <pborzenkov@virtuozzo.com>, Kevin Wolf <kwolf@redhat.com>, Max Reitz <mreitz@redhat.com>

On 06/02/2016 06:58 PM, Denis V. Lunev wrote:
> The problem with excessive flushing was found by a couple of performance
> tests:
>    - parallel directory tree creation (from 2 processes)
>    - 32 cached writes + fsync at the end in a loop
>
> For the first one results improved from 2.6 loops/sec to 3.5 loops/sec.
> Each loop creates 10^3 directories with 10 files in each.
>
> For the second one results improved from ~600 fsync/sec to ~1100
> fsync/sec. Though, it was run on SSD so it probably won't show such
> performance gain on rotational media.
>
> qcow2_cache_flush() calls bdrv_flush() unconditionally after writing
> cache entries of a particular cache. This can lead to as many as
> 2 additional fdatasyncs inside bdrv_flush.
>
> We can simply skip all fdatasync calls inside qcow2_co_flush_to_os
> as bdrv_flush for sure will do the job. These flushes are necessary to
> keep the right order of writes to the different caches. Though this is
> not necessary in the current code base as this ordering is ensured through
> the flush in qcow2_cache_flush_dependency().
>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Pavel Borzenkov <pborzenkov@virtuozzo.com>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Max Reitz <mreitz@redhat.com>
actually this is v2 version of the patchset, missed version number in 
the subject