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Samsung's SM951 PCIe SSD reviewed


Heavyweight horsepower in a featherweight body
— 11:08 AM on June 17, 2015

PC enthusiasts give little credence to arbitrary product segmentation. We want the best parts, and we don't care if they were never meant for our machines. From time to time, this desire has driven us to adopt enterprise-grade hardware with more brawn than consumer-grade gear. It's also taken us into mobile territory, where lower-power components have appealing characteristics for smaller form factors and quieter cooling.

Now, imagine getting the best of both of those worlds—performance that trounces typical desktop parts in a tiny, mobile-friendly package. That's the basic idea behind Samsung's SM951 SSD, which promises speeds over 2GB/s from an M.2 gumstick small enough to pose with Lego minifigs.

The SM951 was never intended to be sold directly to end users. Instead, it was designed for big notebook makers to put into premium systems. But Newegg and other online vendors still stock the so-called OEM drive, making it an intriguing option for anyone seeking wicked-fast PCIe storage. We've tested the SM951 to see whether it's worth appropriating for premium PCs, and the answer isn't entirely straightforward. Let me explain.

Essential details
The SM951 is the follow-up to Samsung's XP941, an older M.2 gumstick with a quad-lane PCIe Gen2 interface. This new generation cranks the same number of lanes up to Gen3 speeds, effectively doubling the aggregate bandwidth to 4GB/s. To put that figure into perspective, consider that the dual-lane Gen2 interface behind the M.2 slots on most motherboards is limited to a mere 1GB/s.

Only a handful of X99 and Z97 boards have "Ultra," "Turbo," or otherwise amped-up M.2 slots with enough bandwidth to fully exploit the SM951. Quad-lane slots should be more common on next-gen Skylake boards due later this summer. In the meantime, the SM951 can be mounted on an M.2 adapter card and plugged into any full-sized PCIe slot.

There are actually two versions of the drive. The initial release uses the familiar AHCI protocol, while a newer variant adheres to the NVM Express standard. Our focus today is on the AHCI model, which is compatible with a broader range of motherboards and available from a wider selection of vendors. We also have the NVMe edition in our labs, but it doesn't work with conventional secure-erase tools, preventing us from wiping the drive between benchmarks. Secure-erasing helps ensure repeatable results, so we're holding off on testing the NVMe version for now.

Like just about every other SSD, including its forebear, the SM951 sports an eight-channel flash controller. The chip's internal cores run at 500MHz, up a healthy 120MHz from the core frequency in the old XP941.

The spec sheet claims end-to-end data protection, but it doesn't promise to preserve in-flight data if the power cuts out unexpectedly. Hardware-accelerated encryption is also missing from the feature set. The drive supports PCIe's L1.2 sleep state, which purportedly lowers power consumption to just two milliwatts, but the savings shouldn't matter in desktop systems.

Samsung says the drive's NAND is fabbed on a "10-nanometer class" process, which means the geometry could be as large as 19 nm. The chips use traditional planar tech rather than the company's multi-layer 3D V-NAND. They pack two bits per cell and 16GB per die.

Capacity Die config Max sequential (MB/s) Max Random (IOps) Price $/GB
Read Write Read Write
128GB 8 x 16GB 2000 600 90k 70k $130 $1.02
256GB 16 x 16GB 2150 1200 90k 70k $240 $0.94
512GB 32 x 16GB 2150 1500 90k 70k $400 $0.78

The 512GB flagship is rated to hit a blistering 2150MB/s with sequential reads and 1500MB/s with writes. Those speeds fall short of the interface's peak theoretical throughput, but they're still faster than what you can expect from competing M.2 drives and plenty of full-sized PCIe SSDs. They're also much better than the random I/O specs, which are average even when compared to SATA SSDs.

Samsung doesn't publish an endurance rating for the SM951, but you probably don't have to worry about burning through the flash. The results of our endurance experiment show that modern MLC drives can survive hundreds of terabytes of writes with barely a scratch. Some can even withstand petabytes of writes without issue.

For a cutting-edge PCIe SSD that's not even supposed to be on the market, the SM951 is surprisingly affordable. The top capacity rings in at under $0.80/GB, which is less than the going rate for Intel's 750 Series, the only other PCIe Gen3 drive on the market right now.

Online listings for the SM951 advertise three-year warranty coverage, but that's between you and the retailer rather than you and Samsung. The warranty is a couple years shy of the five-year coverage typically attached to premium SSDs.

The SM951 is sold as a bare drive, sans accessories and software. It's not supported by the SSD Magician utility Samsung provides for its consumer-oriented SSD, but third-party tools can still read the SMART data. HD Sentinel shows the following attributes:

Between the media wearout indicator and the counters for bad blocks, uncorrectable errors, and flash reserves, there's loads of useful information for monitoring drive health over the long haul. Too bad some of the attributes are obfuscated behind vendor-specific labels.

With the basics covered, we can move on to our performance tests. You won't believe the shocking results on the next page—no, seriously.