Linux / DevOps · Flashcard
What time does the await column in iostat cover?
- AQueue time plus service time — the full wait from issue to completion of a request
- BService time only — the interval the device itself spent working on the request
- CQueue time only — the interval before the device began working on the request
- DCompletion time only — the delay between the device finishing and the caller waking
Why this is the answer
await is the whole round trip a request spends below the block layer, so it rises both when the device is slow and when the queue in front of it is deep — which is why it must be read alongside aqu-sz to tell those two causes apart. Pure device service time is not exported as its own column in current versions. Isolating only the queueing portion would require subtracting service time, which iostat does not do for you. And the wakeup delay after completion is scheduler latency, measured elsewhere entirely.
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