Re: [Qemu-devel] [PATCH 1/1] block: ignore flush requests when storage is clean

[Eric Blake <eblake@redhat.com>]

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On 06/20/2016 09:19 AM, Denis V. Lunev wrote:
> From: Evgeny Yakovlev <eyakovlev@virtuozzo.com>
> 
> Some guests (win2008 server for example) do a lot of unnecessary
> flushing when underlying media has not changed. This adds additional
> overhead on host when calling fsync/fdatasync.
> 
> This change introduces a dirty flag in BlockDriverState which is set
> in bdrv_set_dirty and is checked in bdrv_co_flush. This allows us to
> avoid unnessesary flushing when storage is clean.

s/unnessesary/unnecessary/

> 
> The problem with excessive flushing was found by a performance test
> which does parallel directory tree creation (from 2 processes).
> Results improved from 0.424 loops/sec to 0.432 loops/sec.
> Each loop creates 10^3 directories with 10 files in each.
> 
> Signed-off-by: Evgeny Yakovlev <eyakovlev@virtuozzo.com>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Max Reitz <mreitz@redhat.com>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> CC: Fam Zheng <famz@redhat.com>
> ---
>  block.c                   | 1 +
>  block/dirty-bitmap.c      | 3 +++
>  block/io.c                | 6 ++++++
>  include/block/block_int.h | 1 +
>  4 files changed, 11 insertions(+)

Otherwise seems reasonable, but I'll let others with more experience on
flush semantics chime in.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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