Re: [Xen-devel] questions about the number of pending requests that the host system can detect

[Yuehai Xu <yuehaixu@xxxxxxxxx>]

On Thu, Aug 12, 2010 at 2:04 PM, Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:
>  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
>
The file system I use in the guest system is ext3, which is a
journaled file system. However, I don't quite understand what you said
".. control the write ordering" because the 10 processes running in
the guest system all just send requests, there is no write request.
What do you mean of "barrier writes" here?

Thanks,
Yuehai

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