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Product Review: Samsung SM951 M.2 Drive

Written on August 10, 2015 by Matt Bach
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Table of Contents:
  1. Introduction
  2. Specifications
  3. Performance
  4. Thermal Output
  5. Conclusion
  6. Recommended Reading

Introduction

Over the past few years, M.2 and PCI-E storage devices have become increasingly popular due to the incredible performance they are capable of. While the cost of these drives is often higher than standard SATA SSDs, current M.2 and PCI-E drives can be as much as four times faster and drives that are in development will be ten or more times faster!

The Samsung SM951 is the successor to what was at one time the fastest consumer M.2 drive - the Samsung XP941 (which we reviewed in a previous article). It comes in two flavors - NVMe and AHCI - depending on if you want maximum performance or better compatibility. The NVMe version should be very slightly faster overall, but the AHCI version will have much better mother compatibility if you want to install your OS directly onto the drive.

Specifications

 
Specifications Samsung SM951 512GB Samsung XP941 512GB Intel 750 400GB
Physical Connector M.2 2280 M-key M.2 2280 M-key PCI-E 3.0 x4
Electrical Connection PCI-E 3.0 x4 PCI-E 2.0 x4 PCI-E 3.0 x4
Max Sequential Read 2,150 MB/s 1,170 MB/s 2,200 MB/s
Max Sequential Write 1,500 MB/s 930 MB/s 900 MB/s
Max Random 4KB Read 90,000 IOPS 122,000 IOPS 430,000 IOPS
Max Random 4KB Write 70,000 IOPS 72,000 IOPS 230,000 IOPS
Endurance 150 TBW 150 TBW 219 TBW
Price Samsung SM951 512GB Samsung XP941 512GB Intel 750 400GB
Cost (Newegg) 400 360 390
Cost + Adapter 450 410 390
Cost per GB $0.78 $0.70 $0.98
Cost per GB w/ adapter $0.88 $0.80 $0.98

Physically, the SM951 is identical to the XP941. It uses the same M.2 M-key connector and is exactly the same size (22mm x 80mm). The main difference is that it requires a PCI-E 3.0 x4 slot in order to run at full speed versus the PCI-E 2.0 x4 that is needed for the XP941.

Because of the newer controller and memory chips (and the greater bandwidth available through the PCI-E 3.0 x4 electrical connection), the SM951 is capable of much higher performance than the XP941. In fact, while the sequential read performance is on par with the Intel 750 drives the sequential write performance is significantly higher than the Intel 750 drives. It still cannot compete with the Intel 750's in terms of random read/write IOPS and endurance, but it you just want raw sequential performance the SM951 looks amazing on paper.

Both the AHCI and NVMe version of the SM951 should work as a secondary storage drive on any semi-modern motherboard, although if you want to use it as an OS drive we recommend purchasing the AHCI version. It is supposed to be very slightly slower, but as a tradeoff it should work as a primary OS drive on pretty much any motherboard. The NVMe version, however, can only be used as an OS drive if you install in UEFI mode and only on newer chipsets like Z170, Z97, X99, C602, etc. 

The SM951 512GB is a bit more expensive than the XP941 512GB right now (about $0.78 per GB versus $0.70) but still comes in at about the same price as the Intel 750 400GB even though it is 112GB larger. However, the necessity of a PCI-E adapter with heatsink (due to the extremely high temperature of the controller) raises the effective cost of this drive to be closer to $0.90 per GB. This makes it between two and three times the cost of a standard SATA SSD - although when you consider the fact that you would need to RAID four or more SATA SSDs in order to get the same performance of the SM951 the price isn't actually too bad if you need a very high performance drive.

Performance

We are not going to go into too much of a detailed performance analysis because, frankly, that work has already been done for us by a number of great review sites. Tech Report, Anandtech, The SSD Review, Tweaktown, Legit Reviews, and Toms Hardware have all done extremely through performance analysis of this drive. 

However, just in case something odd comes up we did run our standard harddrive benchmark - CrystalDiskMark 5.01. This benchmark was actually just recently updated to improve the accuracy of the results when benchmarking NVMe drives so it is even more accurate for these ultra fast drives than it was in the past.


As indicated by the advertised specs, the sequential read performance of the SM951 is pretty much on par with the Intel 750 400GB. Also as expected, the sequential write performance is much higher than either the Intel 750 or the Samsung XP941. Interestingly, the random write performance of this drive is only a little higher than the XP941 and no where near the performance of the Intel 750.

One thing we want to point out is that our results are a bit lower than what you will find on other review sites. We used the new Skylake-compatible Asus Z170-A motherboard which for some reason has been giving us a bit lower benchmarking performance for these drives than what we are used to. As far as we can tell the actual performance is identical to other motherboards, but CrystalDiskMark is simply reporting results a bit lower than we expect.

In terms of IOPS, we actually got much higher read and write IOPS than the drive is advertised to be able to achieve. They were still not as high as the Intel 750 400GB (especially in terms of random write 4KB), but it is still very respectable.

Overall, the SM951 is a very balanced drive. Unlike the Intel 750 which has relatively low sequential write performance, both the sequential read and write performance is extremely high. The random read and write performance isn't as good as the Intel 750 but depending on your workload that may not matter to you. 

Thermal Output

We're not going to sugarcoat it - the SM951 runs really, really hot. This is not at all unique to the SM951, however. Every single M.2 drive we have tested that runs at PCI-E x4 speeds runs as hot or even hotter than this drive.


As you can see, the SM951 does run cooler than the XP941 by about 5-7 ºC, but it still reached a peak temperature of 110 ºC. This wasn't after hours of benchmarking in a closed box either. The SM951 hit 100 ºC after only about a minute and 110 ºC after only about three minutes on our open air test bench.

If you notice the slight dips in temperature that occur at about the 150, 190, and 220 second mark, each of those occurred right wen the controller hit 111 ºC for a split second. Since hitting that temperature immediately caused the temperature to drop we believe the drop in temperature to indicate when the drive automatically throttles itself to keep from overheating. Anandtech, Legit Reviews, and many other review sites have shown how quickly the SM951 throttles itself due to high temperatures. 

If you prefer a video over a chart, we also recorded a video showing exactly how hot both the SM951 and XP941 run when doing a simple file copy:

The high temperature of both these drives really make us consider them to not truly be M.2 drives, but rather PCI-E drives that are missing their mounting hardware. There will be some users who will run one of these drives naked in a M.2 slot, but in our opinion the only way to use these drives is to either combine them with a PCI-E adapter that includes a heatsink (we use this adapter ourselves) or otherwise affix a heatsink to the controller chip to help dissapate the heat. Not only is 110 ºC hot enough to impact the longevity of the drive, the fact that the drive throttles around that temperature also means that you will not get anywhere near peak performance unless you add additional cooling via a heatsink.

Conclusion

It is pretty obvious that the SM951 is better than the XP941. It runs a bit cooler (although not enough to make us want to use it naked) and has much higher performance. For us, the main question is how it compares to the Intel 750 drives as those are our current go-to high performance storage drives.

In terms of performance, the SM951 has terrific sequential read and write performance with the sequential write performance in particular being much higher than the Intel 750 drives. The random read and write performance, however, it not nearly as good. This means that if you do a lot of large file copies the Samsung SM951 would be great due to it's high sequential read and write performance. For most users, however, we feel that the much higher random read and write performance of the Intel 750 drives makes them the better drive for the average user. Unless you are doing a straight file copy, you will often be bottlenecked by things like CPU performance long before you would be able to write to the drive at anything even close 1GB/s. This is of course not always going to be true, but in our experience is very often the case.

The Intel 750 drives are a bit more expensive than the Samsung SM951, however the necessity of using a M.2 to PCI-E adapter with a heatsink makes the price difference not as large as it first appears. Yes, you could use the SM951 without an adapter (many X99 and Z170 motherboards have M.2 slots capable of PCI-E 3.0 x4) but with how hot the SM951 runs we would not recommend it. And don't even think about putting one into a laptop unless you are fine with melting the plastic bezels around the drive.

At Puget Systems, we have made the decision to stick with the Intel 750 series as our go-to high performance PCI-E drives. The SM951 is a great drive, but the extra complexity of needing an after-market PCI-E adapter with heatsink and the lower random read/write performance just makes the Intel 750 drives slightly more attractive overall to our customers. If you need high sequential write performance, however, we absolutely would recommend the Samsung SM951... as long as you use a PCI-E adapter with a heatsink that is.

Recommended Reading

If you are interested in M.2 drives, we have a number of other articles you may be interested in:

Tags: Samsung, SM951, XP941, Thermal, Performance
mikeyd

It would be great to know what was done by Puget to get the NVMe version working as a boot drive (Windows installed) on the Asus Z170-A board. I'm not having any luck with this drive on an Asus (Z170) MAXIMUS VIII GENE board.

Posted on 2015-10-21 22:46:14

It should simply be a matter of installing Windows in UEFI mode. These drives don't support booting in BIOS mode (although they work fine as a secondary storage drive in BIOS mode) so you have to use UEFI.

Posted on 2015-10-21 22:52:23
Sgt_Bilko

I've just installed an SM951 AHCI on my 2010 Mac Pro. It generally feels faster, however I am confused because the CPU usage no longer seems to go as high as before when checked in the activity monitor. Previously I could see CPU hit 1600% on certain heavy tasks but have not seen it rise much above 600% since installing the SSD. Is the SM951 limiting my CPU speed?

Posted on 2016-07-30 16:59:07