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.

Every array is inferior to the single HDD in terms of read response time. The 8-disk arrays are not much worse that the latter, though. It is the 4-disk arrays (save for the RAID10) that are slower by about 1 millisecond, which is a considerable gap. As usual, RAID10 are the best by choosing the faster disk in a mirror.

The degraded RAID10 feels good while the degraded arrays with rotated parity are not. They take last places due to the necessity of recovering data from checksums. Of course, the degraded RAID6 without two disks is the worst array here because it has to recover data from two checksums.

It is easy to explain this diagram: the write response time is determined by the total cache of the controller and disks. Thus, the 8-disk arrays are better than the 4-disk ones, and the RAID0 are ahead of the RAID10 because, from a caching standpoint, each mirror of a RAID10 can only be viewed as a single disk. Among the checksum based arrays the 8-disk ones are ahead, too. The RAID5 are better than the RAID6 thanks to processing only one rather than two checksums.

We are impressed by the results of the degraded RAID5 and RAID6, especially of the RAID6 w/o two disks. The overhead associated with the need to read the checksum data and process them is very small. The credit goes to the employed processor. Even with two failed disks the RAID6 has a write response time comparable to that of the single HDD, which is a very good result!