Re: [Qemu-devel] [PATCH 1/3] block: ignore flush requests when storage is clean

[Eric Blake <eblake@redhat.com>]

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On 06/24/2016 09:06 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 unnesessary flushing when storage is clean.

s/unnesessary/unnecessary/ (I pointed this out against v2
https://lists.gnu.org/archive/html/qemu-devel/2016-06/msg05817.html,
which makes me wonder if anything else was missed)

> 
> 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>
> CC: John Snow <jsnow@redhat.com>
> ---

> +++ b/include/block/block_int.h
> @@ -418,6 +418,8 @@ struct BlockDriverState {
>      int sg;        /* if true, the device is a /dev/sg* */
>      int copy_on_read; /* if true, copy read backing sectors into image
>                           note this is a reference count */
> +
> +    bool dirty;
>      bool probed;
>  

Conflicts with the current state of Kevin's block branch (due to my
reordering and conversion of bool parameters); so you'll want to rebase.

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


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