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Re: [Qemu-block] [Qemu-devel] [PATCH 1/3] block: ignore flush requests w

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 1/3] block: ignore flush requests when storage is clean
Date: Fri, 24 Jun 2016 09:31:25 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 06/24/2016 09:06 AM, Denis V. Lunev wrote:
> From: Evgeny Yakovlev <address@hidden>
> 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
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 <address@hidden>
> Signed-off-by: Denis V. Lunev <address@hidden>
> CC: Kevin Wolf <address@hidden>
> CC: Max Reitz <address@hidden>
> CC: Stefan Hajnoczi <address@hidden>
> CC: Fam Zheng <address@hidden>
> CC: John Snow <address@hidden>
> ---

> +++ 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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