Articles: Storage

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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 RAID10 boast a lower response time at reading than the single HDD thanks to effective reading from mirrors. Note that the degraded array is just a little bit worse than the healthy ones. Its response time is very good, too.

The other arrays are behind the single HDD: the 8-disk arrays are half a second slower, and the 4-disk ones are a full second slower. The degraded arrays are at the bottom of the diagram, the RAID6 with two failed disks being the worst. Restoring data from two checksums is not easy.

The more total cache an array has, the lower its response time. This is confirmed by the results of the RAID0 and RAID10 arrays. The checksum-based arrays have problems, though. They are up to five times as slow as the single HDD. This must be another aspect of the problem we have seen in the Database test at the shortest queue depth.

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