IO-wait measures time the CPU could work but everything runnable is blocked on storage. High IO-wait with low CPU usage means the disk — not the processor — is limiting the server.
Typical causes on a hosting VPS: backups, heavy database writes, log floods and noisy neighbours on shared storage. Because ordinary CPU graphs hide it, IO-wait deserves its own line on the chart — when it rises while CPU stays flat, you have your answer.
Frequently asked questions
What IO-wait level is concerning?
Brief spikes are normal; sustained values above 10–20% mean storage is throttling the whole server and the cause is worth finding.
How do I find what causes IO-wait?
Look at what runs during the spike: backups and database activity are the usual suspects. Tools like iotop — or a monitoring snapshot taken at that moment — name the process.