Old question, recently bumped, but felt the existing answers were insufficient.
IOWait definition & properties
IOWait (usually labeled
%wa in top) is a sub-category of idle (
%idle is usually expressed as all idle except defined subcategories), meaning the CPU is not doing anything. Therefore, as long as there is another process that the CPU could be processing, it will do so. Additionally, idle, user, system, iowait, etc are a measurement with respect to the CPU. In other words, you can think of iowait as the idle caused by waiting for io.
Precisely, iowait is time spent receiving and handling hardware interrupts as a percentage of processor ticks. Software interrupts usually are labled separately as
Importance & Potential misconception
IOWait is important because it often is a key metric to know if you're bottlenecked on IO. But absense of iowait does not necessarily mean your application is not bottlenecked on IO. Consider two applications running on a system. If program 1 is heavily io bottlenecked and program 2 is a heavy CPU user, the
%user + %system of CPU may still be something like ~100% and correspondingly, iowait would show 0. But that's just because program 2 is intensive and relatively appear to say nothing about program 1 because all this is from the CPU's point of view.
Tools to Detect IOWait
See posts by Dave Cheney and Xerxes
But also a simple
top will show in
Also, as we are now almost entering 2013, in addition to what others said, the option of simply awesome IO storage devices are affordable, namely SSDs. SSDs are awesome!!!