Fred，
How do you know the disk is completely idle ? How much cache memory do
your controller and disk have ? Why do you think the requests should trigger
action of disk immediately while you don't know what kind of requests are they.
They are reading the same offset of the same block ?
... ....
Before got the answer, you should ask yourself many questions.
Ethan
发自我的 iPad
在 2013-5-26，13:29，Fredrik Tolf <fred...@dolda2000.com> 写道：
> Dear list,
>
> In order to debug I/O performance, I recently wrote a tiny program for
> inspecting /sys/block/$DISK/stat. It works by dumping deltas of the values
> every 100 ms, quite simply (except the queue-length value, for which deltas
> are clearly useless).
>
> Using this, I often see periods during constant I/O loads where there are
> lots of requests in the queue, but the disk is completely idle. They usually
> last somewhere from 0.1 to 2 seconds. Using the aforementioned program, they
> might look, for instance, like this:
>
> 1369545418.8 0 0 0 0 2 0 16
> 4888 134 100 13400
> 1369545418.9 0 0 0 0 0 0 0
> 0 134 100 13400
> 1369545419.0 0 0 0 0 0 0 0
> 0 135 104 13984
> 1369545419.1 0 0 0 0 0 0 0
> 0 135 100 13500
> 1369545419.2 0 0 0 0 0 0 0
> 0 135 100 13500
> 1369545419.3 0 0 0 0 0 0 0
> 0 135 100 13500
> 1369545419.4 0 0 0 0 0 0 0
> 0 135 100 13500
> 1369545419.5 0 0 0 0 0 0 0
> 0 135 100 13500
> 1369545419.6 0 0 0 0 0 0 0
> 0 135 104 14040
> 1369545419.7 2 0 64 672 58 0 1185
> 152512 78 100 11296
>
> I'm sure you all know what the various fields are (except the first, which is
> just a timestamp), so as you can see, there are 135 requests in the queue,
> but no reads or writes happen, in this case, for at least 800 ms.
>
> Is this behavior normal and expected, or is there something wrong here? In
> the latter case, is it my hardware that is failing somehow, or can there be
> some software weirdness that can be tweaked away or bugfixed?
>
> The disk in question is a 640 GB WDC Caviar Green, and it's attached via an
> old Silicon Image 3114 PCI card. Clearly, the hardware is less than optimal,
> but can that explain this behavior? (For the record, the disk does at least
> not report any SMART errors, and there are no errors about in the dmesg.)
>
> The kernel version is 3.7.1, and the scheduler is CFQ.
>
> Thanks for reading!
>
> --
>
> Fredrik Tolf
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