Promise Pegasus2 M4 (4x1TB) Thunderbolt 2 DAS Reviewby Kristian Vättö on September 12, 2014 9:00 AM EST
The Pegasus2 M4: Performance
Let's start with single-drive performance to get the baseline level of performance so we have an idea of what to expect from a 4-drive array. We ran HD Tach on one of the 1TB 5,400rpm Toshiba drives as HD Tach produces a nice graph of performance across all LBAs.
Peak performance is 120MB/s for one drive, so it is reasonable to expect a maximum performance of ~480MB/s from a RAID 0 array. However, by the last LBAs the throughput drops to half that amount.
For array performance, we use Iometer due to its flexibility. Sequential performance is tested with a transfer size of 2MB at queue depth of one and the test runs for one minute, while 4KB random performance is tested at queue depth of three for three minutes. All tests are run with an 8GB LBA space, so we are looking at the best case performance here – the HD Tach graph above gives you an idea of how the performance degrades as the array is filled.
|Promise Pegasus2 M4 4TB (4x1TB) Performance|
|2MB Sequential Read (QD1)||2MB Sequential Write (QD1)||4KB Random Read (QD3)||4KB Random Write (QD3)|
|Pegasus2 M4 (RAID 0)||480.6MB/s||466.81MB/s||0.95MB/s||4.42MB/s|
|Pegasus2 M4 (RAID 5)||355.5MB/s||356.0MB/s||0.94MB/s||0.67MB/s|
|Pegasus2 M4 (RAID 10)||408.4MB/s||240.5MB/s||1.11MB/s||2.15MB/s|
In a RAID 0 configuration, the M4 manages up to 480MB/s, which is where the hard drive becomes the bottleneck. That is far from the maximum performance of Thunderbolt 2 (20Gbps or 2.5GB/s) but over 100MB/s faster than what USB 3.0 is typically capable of providing.
With RAID 5 the write performance takes a hit because of the parity writes (you can only write data to three disks simultaneously as the fourth one will be writing parity data), but read speed is also affected. It could be a limitation of the RAID controller itself because some RAID controllers do not tend to work as well with RAID 5 because of the processing power required for parity calculation. RAID 10 also experiences similar performance loss as in theory RAID 10 should provide the same read performance as RAID 0, but that is not the case with the M4 and its RAID implementation.
I wanted to see what the M4 is capable of when fitted with faster hardware, so I took four 256GB SSDs and configured them in a RAID 0 array. I ran the same tests as above but added a test with queue depth of 16 to see the maximum throughput of the M4.
|Promise Pegasus2 M4 1TB (4x256GB SSD) Sequential Performance|
|2MB Sequential Read (QD1)||2MB Sequential Read (QD16)||2MB Sequential Write (QD1)||2MB Sequential Write (QD16)|
|Pegasus2 M4 (RAID 0 - SSD)||579.1MB/s||719.0MB/s||532.6MB/s||617.0MB/s|
|Pegasus2 M4 (RAID 0 - HDD)||480.6MB/s||478.4MB/s||466.81MB/s||456.4MB/s|
With SSDs the M4 takes more advantage of Thunderbolt, although performance is still not that great. Four SATA 6Gbps SSDs should be capable of ~2GB/s in sequential read, so it is clear that the SATA/RAID controller (or the RAID controller) is limiting performance. For mechanical storage that is not an issue but we can see why Promise is not offering an SSD configuration – it simply would not be worth the extra cost since the performance upgrade is rather insignificant, at least for video work where random accesses are not critical.
|Promise Pegasus2 M4 1TB (4x256GB SSD) Random Performance|
|4KB Random Read (QD3)||4KB Random Write (QD3)|
|Pegasus2 M4 (RAID 0 - SSD)||16.75MB/s||30.1MB/s|
|Pegasus2 M4 (RAID 0 - HDD)||0.95MB/s||4.42MB/s|
Random performance is obviously multiple times better with SSDs but nowhere near what it should be with four SSDs in RAID 0. Even a single SATA 6Gbps SSDs scores around 90MB/s in random read and easily over 200MB/s in random write, so the SATA/RAID controller appears to be the bottleneck once again.