Hunting I/O Bottlenecks with iostat

The October 2004 “Tech Support” column (http://www.linuxmagazine.com/2004-10/tech_support_01.html) showed you how to track down performance bottlenecks using vmstat. This month, let’s take a closer look at input/output (I/O) issues that you may have identified using vmstat.

The iostat command monitors system I/O device loading by observing the time devices are active in relation to their average transfer rates. The iostat command generates reports that can be used to modify your system configuration to better balance the I/O load between physical disks or to let you know when you have reached the threshold of your current disk subsystem.

Running iostat with no arguments generates a report that contains information since the system was booted. You can provide two optional parameters to change this:

$ iostat [ interval [ count ] ]

The interval parameter specifies the amount of time in seconds between each report. You can specify the count parameter in conjunction with the interval parameter and control how many reports are generated before iostat exits. When using these arguments, the first report contains information since the system was booted, while each subsequent report covers the time period since the last report.

By default, iostat generates two reports, one for CPU utilization and one for device utilization. You can use the –c option to get just the CPU report or the –d option to get just the device report. Here is the default output from iostat: