What is IOWait relative to the sar command, and when should I be concerned?

Question

What is IOWait relative to the sar command, and when should I be concerned?

Answer

Low to moderate amounts of IOWait can be acceptable for applications that run in the background and are not time-sensitive. However, for web applications, where load times are often expected to be sub-1 second and lower, even a small amount of IOWait can make a noticeable difference.

If you notice a decline in the performance of your application and you can see that IOWait is above 4 percent, you should consider reaching out to a systems administrator that has the skills, training, and expertise to determine the cause for the IOWait, and what you can do to lower it.

Very high amounts of persistent IOWait, such as 60 percent, almost always correlate with serious, widespread issues on the server. This indicates that the demands placed on your server's disks have exceeded the threshold of the capabilities of the disks. You should recruit the assistance of a systems administrator to determine the best course of action to resolve the issue.

Additional resources

sar(1) - Linux man page