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Re: [Qemu-block] [PATCH 1/1] block: ignore flush requests when storage i

From: Denis V. Lunev
Subject: Re: [Qemu-block] [PATCH 1/1] block: ignore flush requests when storage is clean
Date: Tue, 21 Jun 2016 10:41:18 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1

On 06/21/2016 10:32 AM, Paolo Bonzini wrote:

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.

Really interesting point, I have missed it. Though this case
is exactly one which we want to optimize. 2nd flush is
unnecessary and should not be sent, BUT you perfectly
correct it must return later than the first to the guest.

Have to rework. Nice catch!


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