Iostat

The UNIX tool Iostat is useful for examining the throughput of various disk resources. If one or more disks have high throughput, understanding where that throughput is coming from is vital. If there are spare CPU cycles, IO is often the culprit.

The specifics of Iostat output vary slightly from system to system. Here is an example from a Linux machine which slows a relatively light load:

(The first set of output is cumulative data since the machine was booted)

Device: tps   Blk_read/s  Blk_wrtn/s  Blk_read   Blk_wrtn 
dev8-0  13.50 144.09      122.33      346233038  293951288
...
Device: tps   Blk_read/s  Blk_wrtn/s  Blk_read   Blk_wrtn
dev8-0   4.00 0.00        96.00       0        96