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

[Evgeny Yakovlev <eyakovlev@virtuozzo.com>]

On 24.06.2016 18:31, Eric Blake wrote:
> 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)

Yeah, i fixed that but messed up committing a change in commit message. 
Will be fixed in rebased version.

>> 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.
>

Ok