What is iowait?
iowait is the percentage of time a CPU core spent idle while waiting for disk IO to complete. It is a critical signal for understanding whether disk is slowing down your system.
Evidence to look for
iowait appears in top, vmstat, iostat, and /proc/stat
Values above 5-10% on busy systems may indicate disk bottlenecks
High iowait combined with high CPU can mean threads are piling up on disk waits
What it usually means
iowait does not mean the CPU is 'doing IO work.' It means the CPU had nothing else to do while waiting for disk. On a busy system, high iowait means disk is becoming a bottleneck. On an idle system, it can be normal.
What to do next
- 1Check disk utilization and await times when iowait is elevated
- 2Move to faster storage if disk is saturated
- 3Reduce the number of concurrent disk-heavy operations
See what's causing your CPU
cpum.ai turns CPU, process, disk, and memory signals into plain-English explanations with evidence.
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