iostat can iteratively report terminal and disk I/O activity, as well as CPU utilization. The first report is for all time since a reboot and each subsequent report is for the prior interval only.
In order to compute this information, the kernel maintains a number of counters. For each disk, seeks and data transfer completions and number of words transferred are counted; for terminals collectively, the number of input and output characters are counted. Also, at each clock tick, the state of each disk is examined and a tally is made if the disk is active. The kernel also provides approximate transfer rates of the devices.
iostat's activity class options default to tdc (terminal, disk, and CPU). If any activity class options are specified, the default is completely overridden. Therefore, if only -d is specified, neither terminal nor CPU statistics will be reported. The last disk option specified (either -d or -D) is the only one that is used.
Milliseconds per average seek is an approximation based on the disk (not the controller) transfer rate. Therefore, the seek time will be over-estimated in systems with slower controllers.
iostat only provides statistics on the first ten disk drives, all others are ignored.
Created by unroff & hp-tools. © by Hans-Peter Bischof. All Rights Reserved (1997).
Last modified 21/April/97