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From: "Denis V. Lunev" <den@openvz.org>
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Date: Tue, 21 Jun 2016 11:17:21 +0300
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Subject: Re: [Qemu-devel] [PATCH 1/1] block: ignore flush requests when
 storage is clean
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To: Kevin Wolf <kwolf@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, Fam Zheng <famz@redhat.com>, Evgeny Yakovlev <eyakovlev@virtuozzo.com>, Max Reitz <mreitz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>

On 06/21/2016 10:45 AM, Kevin Wolf wrote:
> Am 21.06.2016 um 09:32 hat Paolo Bonzini geschrieben:
>>
>> On 20/06/2016 17:19, Denis V. Lunev wrote:
>>> +    /* Check if storage is actually dirty before flushing to disk */
>>> +    if (!bs->dirty) {
>>> +        goto flush_parent;
>>> +    }
>>> +    bs->dirty = false;
>>> +
>> This should be cleared after the flush is complete.  If you have
>>
>>      write begin
>>      write end
>>      flush #1 begin
>>      flush #2 begin
>>
>> Then the second flush must only return after the first has finished.
> I think clearing bs->dirty after the flush completion wouldn't
> necessarily be right either if there are concurrent writes in flight, as
> only completed writes are guaranteed to be flushed by it.
>
> Kevin
this is not a problem if flush 2 will return after flush 1.
This will mean that all writes prior to both flushes
will land to the disk.

Keeping this in mind dirty should be cleared before
flush operation start.

Den