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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