The M.2 form factor has quickly established itself as the most popular choice for PCIe SSDs in the consumer space. The small size easily fits in to most laptop designs, and the ability to provide up to four lanes of PCI Express accommodates even the fastest SSDs. By comparison, SATA Express never caught on and never will due to its two-lane limitation. And the more recent U.2 (formerly SFF-8639) does have traction, but has seen little adoption in the client market.

Meanwhile, although M.2 has its perks it also has its disadvantages, often as a consequence of space. The limited PCB area of M.2 can constrain capacity: Samsungs single-sided 950 Pro is only available in 256GB or 512GB capacities while the 2.5" SATA 850 Pro is available in up to 2TB. And for Intel, the controller used in their SSD 750 is outright too large for M.2, as its wider than the most common M.2 form factor (22mm by 80mm). Finally and most recently, as drive makers have done more to take advantage of the bandwidth offered by PCIe, a different sort of space limitation has come to the fore: heat.

When testing the Samsung SM951 we found that our heavier sustained I/O tests could trigger thermal throttling that would periodically restrict the drives performance. We also had a brief opportunity to run some of our tests on the SM951 using the heatsink from Plextors M6e Black Edition. We found that extra cooling made noticeable differences in performance on some of our synthetic benchmarks, but our more realistic AnandTech Storage Bench tests showed little or no change. But other than the quick look at the SM951, we havent had the chance to do a thorough comparison of how cooling affects high-performance M.2 drives, until now.