I/O wait is simply one of the indicated states of your CPU / CPU cores. It’s important to note that iowait can, at times, indicate a bottleneck in throughput, while at other times, iowait may be completely meaningless. It’s possible to have a healthy system with high iowait, but also possible to have a bottlenecked system without iowait. If nothing is pending, then the ‘idle’ counter is incremented. If there are, then the ‘iowait’ counter is incremented. If the CPU is idle, the kernel will ascertain any pending I/O requests (i.e., SSD or NFS) originating from the CPU. “Idle” CPU means there is no workload present while, on the other hand, “wait” (iowait) indicates when the CPU is waiting in an idle state for outstanding requests. Note that “idle” and “wait” are not the same. Of these, the user, system, idle, and wait values should add up to 100%. When using Linux top and other tools, you’ll notice that a CPU (and its cores) operate in the following states: us (user), sy (system), id (idle), ni (nice), si (software interrupts), hi (hardware interrupts), st (steal) and wa (wait). “iowait shows the percentage of time that the CPU or CPUs were idle during which the system had an outstanding disk I/O request.” – iostat man page.

(Source: man sar) The top man page gives this simple explanation: “I/O wait = time waiting for I/O completion.” In other words, the presence of I/O wait tells us that the system is idle when it could be processing outstanding requests. I/O wait (iowait) is the percentage of time that the CPU (or CPUs) were idle during which the system had pending disk I/O requests. I/O wait applies to Unix and all Unix-based systems, including macOS, FreeBSD, Solaris, and Linux. What is I/O wait? How does I/O wait affect Linux server performance? How can we monitor and reduce I/O wait related issues? Continue reading for the answers to these questions.

The cause? Storage I/O bottleneck was initially hinted at by a consistently high iowait and later confirmed with additional investigation. This resulted in slow page loading, timeouts, and intermittent outages. During our support call, they reported load spikes of 60 to 80 on their 32 CPU core system. I/O wait came up in a recent discussion with a new client.

Ubuntu cpu usage widget