Promise Pegasus2 M4 (4x1TB) Thunderbolt 2 DAS Review
by Kristian Vättö on September 12, 2014 9:00 AM EST- Posted in
- Storage
- Thunderbolt
- m4
- Promise
- DAS
- Thunderbolt 2
- Pegasus2
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.
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Kristian Vättö - Sunday, September 14, 2014 - link
I hope you understand that there is no central AnandTech office with every possible gadget and tool at our disposal. I don't have any other Thunderbolt enclosures since this was my first TB review, so I wouldn't have anything to compare the M4 to. Anand used to run our Thunderbolt coverage since he had the best tools for that, and it will take some time before we get everything migrated.melgross - Sunday, September 14, 2014 - link
You couldn't get the site to cough up $40? Or use your smartphone. There are plenty of apps that measure sound levels. I can't speak to the accuracy of Android apps because there are so many phone models out there that they may not all be calibrated. But the ones for the iPhone, such as Audio Tools, also available for Android, is. Mine measures within 1db of my lab meter.melgross - Sunday, September 14, 2014 - link
I forgot to mention that using an app such as Audio Tools is better because you can also use the very accurate spectrum analyse tool. This is important for useful noise testing because all noise is not equal. Low frequency noise doesn't sound as loud to us as mid frequencies and higher frequencies do. This shows a live reading chart. Also great for detecting hum, and other spikey noises. You can also save the chart and put into the review.bigboxes - Monday, September 15, 2014 - link
I don't comment on everything. However, Anand is gone. You've got to stop using him as an excuse as to why you don't do someting. Move on. It's almost painful reading your comments. Yes, Anand is gone. Now, it's time for you to move on.melgross - Sunday, September 14, 2014 - link
Is like them to spend $40 and buy a meter which is accurate to within a couple of dbx such as the Radio Shack model. At least we would get some idea of how loud it is. It doesn't need to be exact.Kristian Vättö - Sunday, September 14, 2014 - link
I definitely want to test with a Mac because obviously that is the target market of Thunderbolt products. Unfortunately I don't have one at my disposal and since I have to spend my own money on it, I want to make sure that what I buy is worth it in my opinion (I currently have a 2010 MBA, but it's still serving me fine so I haven't seen a need to upgrade yet).colinstu - Saturday, September 13, 2014 - link
I'd love to see four WD1000CHTZ thrown in this puppy and see how it compares. (10k RPM, SATA 1TB drives). They're 9.5mm too just like the ones used here. Or take a WD1000DHTZ and take the 2.5->3.5 "icepack" off.jseliger2 - Sunday, September 14, 2014 - link
<i>While there are arguably cheaper and larger external 3.5" hard drives around</i>There was just a thread about this issue over at Ars Technica: http://arstechnica.com/civis/viewtopic.php?f=19&am... , and I"m curious about what those other external drive bays might be. Arsians list an OWC option, a Western Digital version, and a Drobo version. They're all costly. Are there other alternatives?
Kristian Vättö - Sunday, September 14, 2014 - link
I meant external HDDs in general, including USB 3.0 ones. If you are just looking for a single drive, then USB 3.0 is fine because a 3.5" HDD cannot saturate the USB 3.0 interface anyway. It's only when you start doing RAID (or SSDs) that Thunderbolt becomes beneficial.jseliger2 - Sunday, September 14, 2014 - link
I meant external HDDs in generalOh—I see.
I'm still curious about the multi-bay enclosure space, since those multi-bay enclosures still seem to be shockingly expensive. Perhaps that will remain true: http://www.anandtech.com/show/8529/idf-2014-where-... , but I'd like it not to be. As they say in that Ars thread, the four-bay Thunderbolt Drobos are about twice the price of the equivalent USB 3 model.