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Re: [Xen-devel] questions about the number of pending requests that the

To: Yuehai Xu <yuehaixu@xxxxxxxxx>
Subject: Re: [Xen-devel] questions about the number of pending requests that the host system can detect
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Thu, 12 Aug 2010 11:04:52 -0700
Cc: yuehai.xu@xxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx, yhxu@xxxxxxxxx
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User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.7) Gecko/20100720 Fedora/3.1.1-1.fc13 Lightning/1.0b2pre Thunderbird/3.1.1
 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

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