From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: questions about the number of pending requests that
	the host system can detect
Date: Thu, 12 Aug 2010 11:04:52 -0700
Message-ID: <4C6437C4.3040908@goop.org>
References: <AANLkTi=3d=+J2WNbBM5rLu_MfpjA_3OXhm8qOkCf_sLg@mail.gmail.com>
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To: Yuehai Xu <yuehaixu@gmail.com>
Cc: yuehai.xu@gmail.com, xen-devel@lists.xensource.com, yhxu@wayne.edu
List-Id: xen-devel@lists.xenproject.org

  On 08/11/2010 08:42 PM, Yuehai Xu wrote:
> However, the result turns out that my assumption is wrong. The number
> of pending requests, according to the trace of blktrace, is changing
> like this way: 9 8 7 6 5 4 3 2 1 1 1 2 3 4 5 4 3 2 1 1 1 2 3 4 5 6 7 8
> 8 8..., just like a curve.
>
> I am puzzled about this weird result. Can anybody explain what has
> happened between domU and dom0 for this result? Does this result make
> sense? or I did something wrong to get this result.

If you're using a journalled filesystem in the guest, it will be need to 
drain the IO queue periodically to control the write ordering.  You 
should also observe barrier writes in the blkfront stream.

     J