Yenya's World

Fri, 24 Mar 2006

HDD utilization

I am a statistics freak. At our FTP server, I have a statistics page with various measurements. These has many times proven themselves to be useful. The most interesting data can be collected when there is a high load on the server (which is otherwise very hard to simulate). On our FTP server, the high load occurs mainly during Fedora Core releases, as we run an official Fedora mirror. Yesterday I have found something interesting in the stats:

disk utilization of /dev/sdg disk utilization of /dev/sdh

The first image shows the utilization of /dev/sdg, the second one shows the utilization of /dev/sdh. By "utilization" I mean the percentage of time when there was at least one pending request to the drive. I measure this in order to see whether the HDDs are the bottleneck of the server. The server has eight identical drives, WD2500JB (altough of different age and firmware revisions because of replacement of the faulty drives over the time).

As you can see, the /dev/sdg has approached the 100% utilization several times, while the other drive was nowhere near to this utilization (the graphs for the other six drives are similar to /dev/sdg). But the load on both drives is similar - both have the same number of sectors per second read and written (as it can be seen on the HDD stats page - I will not copy the images here). Both drives are members of the same SW RAID volumes. I have tried to watch the iostat -x 5 output, and it seems that /dev/sdg has not only a higher utilization, but also the longer request wait time and request service time, while the amount of sectors read and written is about the same. The sdg drive is a year newer than sdh.

So, any idea about what might be the cause of the higher utilization of one drive out of eight almost identical ones?

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