Evaluating the current disk performance with iostat

Due to the disparity in speed between storage and RAM, one of the first signs of distress that a DBA will observe is directly related to disk utilization. A badly written query, an unexpected batch-loading process, a forced checkpoint, overwhelmed write caches - the array of things that can ruin disk performance is vast.

The first step in tracking down the culprit(s) is to visualize the activity. The iostat utility is fairly coarse in that it does not operate at the process level. However, it does output storage activity by device and includes columns such as reads or writes per second, the size of the request queue, and how busy it is compared to its maximum throughput.

This allows us to see the devices that are actually slow, busy, or overworked. Furthermore, we can combine this information with other methods of analysis to find the activity's source. For now, let's explore the tool itself.

Getting ready

As iostat is part of the sysstat package...