Typically multi-bay external storage devices tend to utilize 3.5" drives due to the lower cost and higher capacities. The downside, however, is that 3.5" drives are physically larger and heavier, which makes a multi-bay enclosure rather difficult to move around on a regular basis. To fix this, Promise is offering a 4-bay 2.5" RAID solution called the M4.

Aside from capacity, the M4 offers everything that the R4 does as you get hardware RAID 5 and two Thunderbolt 2 ports for daisy-chaining. The weight comes in at almost one third of the R4's weight and the dimensions are considerably smaller too, which makes the M4 a lot more portable than the rest of the Pegasus2 lineup. Sadly Thunderbolt 2's ten watts of power is not capable of powering the M4, so it is not a fully portable solution like regular external hard drives are.

The M4 is available for $999 in the Apple Online Store and the target market for the M4 and the whole Pegasus2 family is video professionals. Promise markets the M4 as a solution that offers portability for over an hour of uncompressed 4K footage. While there are arguably cheaper and larger external 3.5" hard drives around, the M4 provides redundancy via RAID 5, 6 and 10, which is more or less a must for professional video editing because data loss could end up being very expensive.

Our review unit shipped with four 1TB 5,400rpm Toshiba hard drives. These are 9.5mm i.e. two-platter drives, so we are not dealing with super high density here. Promise told us that they are not offering 4x1.5TB or 4x2TB configurations due to price sensitivity as $999 is quite expensive to begin with, although I am not sure if I agree because I could see video professionals paying more for increased capacity. In the end, 4TB is not that much if you deal with 4K video.

Fortunately Promise has made hard drive swaps convenient as pressing the button on the bay will free the lever, which you simply pull to get the drive out. The drives are attached to the bays by four standard hard drive screws, so any 2.5" drive will work. Officially Promise only guarantees compatibility with the Toshiba drive, although the user manual suggests that the drive does not have to be the same make and model.

Getting inside the M4 is fairly easy. There are a few screws that need to be removed until the top comes off and you end up having access to the PCB along with the rest of the components (PSU, fan, etc.). The RAID controller is covered by the heat sink, so I do not have a photo of it, but I was told that the silicon itself is from PMC with custom Promise firmware. A quick look at PMC's RAID controller lineup suggests that the silicon is the PM8011 SRC 8x6G, which is an 8-port SATA/SAS 6Gbps controller with a PCIe 2.0 x8 interface. 

Like many Thunderbolt devices, the M4 has two Thunderbolt 2 ports for daisy-chaining. The Thunderbolt controller is Intel's DSL5520 with two Thunderbolt 2 ports (i.e. four channels) and it connects to the RAID controller through a PCIe 2.0 x4 interface. Intel lists the bulk price as $9.95 on their ARK site and the TDP is 2.8W.

Test Setup

Unfortunately I do not have a Mac with Thunderbolt, so the results and analysis are limited to a Windows based system. Based on what we have talked with manufacturers, there is some difference in performance between Thunderbolt in Windows and OS X. A part of that comes from the fact that in PCs, the Thunderbolt controller is connected to the PCIe lanes from the PCH, whereas in Macs they come directly from the CPU. The Windows drivers are also not as good as the native OS X drivers, which I guess is not a surprise given that Apple has always been the biggest supporter of Thunderbolt. Either way, the results should represent performance under both OSs as long as we are not close to saturating the interface.

• Thanks to Intel for the Core i7-4770K CPU
• Thanks to ASUS for the Z87 Deluxe motherboard and ThunderboltEX II/DUAL adapter
• Thanks to Corsair for the Vengeance 16GB DDR3-1866 DRAM kit, RM750 power supply, Hydro H60 CPU cooler and Carbide 330R case

The Pegasus2 M4: Software

Since my only Thunderbolt-enabled system is Windows based, our look at the software is limited to the Windows version, but our original Pegasus R6 review has screenshots from the OS X software.

For managing the M4 and other products, Promise offers WebPAM PRO software. It is web-based software that runs in the browser and upon launching it will ask for your Windows user's credentials before you are granted access to WebPAM PRO. The dashboard view just gives a quick overview of the device and its status.

The device tab offers a real-time graphical view of the device and its components. Clicking either the device graphic or the list on the upper right corner will give more details about the enclosure, controller, and physical drives. Below is a gallery with additional screenshots that shows what exactly the WebPAM PRO allows you to monitor.

Creating the array can be done under the storage tab and once again you are presented with a real-time graphical view of the device. To select the drives you want in the array, simply click the drive bay and it will turn blue. Once you have selected the bays you want, the submit button will lead you to the next screen.

The final step is to set the RAID level and other specific aspects. RAID 5 is the default RAID level in the M4 as it comes with a hardware RAID controller, but RAID 0 and RAID 10 are supported as well.

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.

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.

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.

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.

Final Words

It is clear that the Pegasus2 M4 is a niche product. Any user that does not require portability will be better served by the R4, which is a similar 4-bay Thunderbolt 2 device but with 3.5" hard drives instead of 2.5" drives. As a result it is physically larger but should provide better performance since 3.5" hard drives tend to be faster than their 2.5" siblings. We have not tested the new R4, but we did test the original R6 when it was released and the performance was better than what the M4 offers. The R4 is also more cost efficient and it retails for $1,499 at the Apple Online Store, but that includes 8TB of data so that works out to be $187.50 per terabyte whereas the M4 costs $250 per terabyte.

Furthermore, a single 3.5" external hard drive can beat the M4 in capacity while costing a fraction of the M4's price, and it provides the same or even higher level of portability. That leaves redundancy and performance as the M4's advantages. Since the M4 supports RAID 5 and RAID 6, it can withstand one or two hard drive failures without losing any user data. The performance is also much better than what you can get from single 3.5" drives, which usually max out at around 200MB/s, whereas the M4 manages over 350MB/s in RAID 5 configuration.

I did notice one irritating thing in the M4, however. The fan in the M4 makes a fairly loud noise even when the device is idling.  Unfortunately I do not have a proper decibel meter to provide an objective measure of the noise, but WebPAM PRO showed the fan speed to be 2,400rpm, which certainly sounds high. I would not say the noise is too loud to distract me while working, but it is clearly distinguishable over the noise that the three desktops in my office create.

Another thing I have a slight problem with is the lack of additional capacity points. 4TB is not much for a video professional, especially if 4K video is being stored, and 1.5TB and 2TB 2.5" 9.5mm hard drives are not that expensive anymore. A 2TB 2.5" 9.5mm hard drive costs $127 online while a similar 1TB drive costs $65, so the price is exactly double. It is understandable that Promise must have high margins because these are not mainstream products that sell in high volumes and thus I can see the cost sensitivity issue with higher capacities, but a 4x2TB configuration at $1,499 would still leave Promise with a ~50% margin on the hard drives.

All in all, the M4 targets a fairly small niche. I can only think of video professionals (maybe photo and audio to some extent) that would see the value in the M4 because if you do not need portability, redundancy, and high performance, there are better options in the market. However, if you need all three (e.g. for on-set video editing), the M4 does the job.