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 2:16 PM, Yuehai Xu <yuehaixu@xxxxxxxxx> wrote:
> 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
>
I am sorry for the missing word, the requests sent by the 10 processes
in the guest system are all read requests.

Thanks,
Yuehai

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