Analyzing I/O Wait Latency with Ftrace and Block Tracing

Your application is slow. top shows 60% iowait. You assume the disk is hammered, but here’s the thing—iowait is a CPU accounting trick, not an I/O performance metric. A CPU showing high iowait just means it’s idle with tasks blocked on I/O. The actual I/O might be fine; you just have nothing else to run.

We traced a PostgreSQL checkpoint storm at 3 AM. T…