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Request Type.

Write IOs are often returned to the host once the block is written in the controller's NVRAM. Later, they are flushed to disk during the destage process. However, read IOs may need to go to disk more often. Similarly, two requests from a single stream may have widely varying latencies if one hits in the cache and the other misses. In certain RAID systems [5], writes may take four times longer than reads due to parity reads and updates. In general, IOs from a single stream may have widely-varying response times, affecting the latency estimate. Fortunately, a moving average over a sufficiently long period can absorb such variations and provide a more consistent estimate.

Ajay Gulati 2009-01-14