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Disk Response Time

IOMeter is sending a stream of requests to read and write 512-byte data blocks with a request queue depth of 1 for 10 minutes. The disk subsystem processes over 60 thousand requests, so the resulting response time doesn’t depend on the amount of cache memory.

The read response time of the RAID10 arrays is lower than that of the single HDD thanks to effective reading from the mirror pairs. The 4-disk is 1 millisecond faster, which is a large gap considering that the single HDD has a read response of 6 milliseconds. We can’t but applaud the programmers of the controller’s firmware, yet it is unclear why it is the 4-disk rather than 8-disk array that turns in the best result.

The arrays of the other types are worse than the single HDD, the 8-disk ones being better than the 4-disk ones. This is all normal, however, because the controller cannot be absolutely transparent in terms of response time. It is only unclear why the RAID0 has the maximum response time among the different array types.

Theoretically, the write response time is determined by the size of the combined cache of the controller and HDDs: the requests go into the cache and then scatter among the disks. Here, the arrays all comply with the theory, excepting the 4-disk RAID0 whose response time is too high (it should equal the 8-disk RAID10’s). The controller’s write response is overall far from record-breaking: the competitor products come with more cache.

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