Our dual-core Opteron test system with Areca’s ARC-1220 controller provided a great platform to cross-test RAID 5 and RAID 6 setups with three to eight hard drives. We found that the controller has its limits at just under 500 MB/s, which provides sufficient bandwidth for most RAID 5 and all RAID 6 scenarios. RAID 5 read transfer performance is close to maxing out with only six drives. With RAID 5, it thus makes a lot of sense to heed the minimum transfer rates to make sure that the storage subsystem’s performance does not drop below certain performance numbers. More than six drives make perfect sense if you need to guarantee certain throughput levels.
Write transfer performance is bottlenecked by the performance of Areca’s hardware XOR engine, which is an Intel IOP332. While 384 MB/s are possible when reading eight drives in RAID 5, the maximum write transfer rate of such a configuration was 186 MB/s. In case of RAID 6 scenarios, write performance is even lower due to the double parity calculation that is necessary. We found the difference between six and eight hard drives to be negligible. RAID 6 is really safe, but it’s not fast.
The average access times of our RAID arrays suffered considerably as the drive count went up, going all the way from the 14 ms of a single Samsung HM321KJ to as much as 31.8 ms for a RAID 6 array with eight hard drives. This also has a noticeable impact on I/O performance, which was somewhat disappointing at very short command queue depths. However, RAID 6, and especially, RAID 5 arrays provide awesome I/O performance that scales very linearly when compared to a single hard drive, offering way beyond 1,000 I/O operations per seconds with up to eight drives.
We believe that RAID 6 only makes sense when frequent, high-bandwidth access is not required. Think of using a RAID 6 array as a primary backup, where you could store your company’s project data at the end of each day (do not use it as your only backup, though). RAID 5 offers the most-balanced performance (while taking access time and throughout into account) with four or five hard drives. More drives might increase the throughput, but they also increase access time. Finally, we recommend against using a large number of hard drives, because the chance of having to deal with a degraded RAID array increases with every drive you add. If you need a high-performance array, get a reasonable amount of high-performance hard drives. If capacity is your main concern, opt for fewer drives that also offer more capacity. In the end, fewer drives will also have a positive impact on power consumption Compare Prices on RAID Controllers.
Please also have a look at Part 1 of the RAID Scaling Charts here for more information.