Atto/iStoragePro Test Results for RAID-6 Striping + Dual Parity

iStoragePro with Atto R380 using Oct 26 2010 firmware

RAID-6 scalability is near-linear through 8 drives for reads, but flatlines at 7 drives for writes. This performance is consistent with the RAID-5 results and RAID-0 results .

As one might hope, RAID-6 with its two parity drives achieves performance very similar to RAID-0 with 2 more drives. In other words, 8 drives for RAID-6 is about as fast as 6 drives for RAID-0, though it could be somewhat less as the same amount of data is added.

Since RAID-0 is seen to be non-scalable past 6 drives, from a reliability perspective, an 8-drive RAID-6 configuration is a far better choice for its resilience: up to two drive failures can be tolerated.

Scalability of RAID-6 striping + dual parity with 4...8 drives

Speed compared to RAID-5

The results of a DiskTester fill-volume test are shown below.

Here, the RAID-5 results are using 7 drives plus a hot spare, so the same total capacity is at play( (12GB).

The Atto R380 card throttles RAID-5 write performance to about 635 MB/sec. Oddly, RAID-6 writes are slightly faster than RAID-5 writes, which are throttled to about 655MB/sec. Faster performance with two parity drives is unexpected.

Since hard drives vary in speed over the area of the drive, the flatlined write performance continues until the drive speed declines below the throttling effect.

Click for a full-size graph.

DiskTester fill-volume
MB/sec filling the entire drive, then reading the files