Samsung SM951

Another blisteringly fast, but slightly easier-to-find, Samsung M.2 PCI Express SSDA little while back, we looked at Samsung’s 512GB XP941 M.2 PCIe SSD – a rare beastie to get hold of, but worth tracking down for its stunning performance. Well, stunning that is if you could run it at its full x4 PCIe speed. But now we have our hands on its successor, the slightly easier-to-obtain SM951.

The SM951 M.2 drive first saw the light of day at Samsung’s annual shindig, the SSD Global Summit, which took place in Seoul, South Korea, in July last year. When it was first announced, the company claimed it would be the world’s first NVMe SSD for the PC, but that idea sadly got kicked out of the window by the powers that be, as the drive only supports the good old AHCI architecture. (Samsung has just announced that it’s started mass production of the NVMe version of the drive, called the SM951-NVMe).Like the XP941, the SM951 is handled by the Korean firm’s OEM branch, which is what makes getting hold of one without buying some sort of device with it preinstalled a tad tricky.The drive comes in three capacities, the entry level 128GB, the 256GB drive and the flagship 512GB unit. There’s no 1TB drive as per the original announcement and all are the M.2 2280 format (22mm wide, 80mm long).The XP941 used a 3-core Samsung S4LNO53X01 controller with a PCIExpress 2.0 x4 interface. The SM941, meanwhile, uses a Samsung S4LN058A01 controller, again a 3-core chip, but with a PCI Express 3.0 x4 interface. So what does moving from 2.0 to 3.0 PCIe do then? Well, in theory it doubles the bandwidth – you only have to look at the quoted sequential read/write performance figures of the drives to see the advantage of the 3.0 interface. The 256GB XP941 is quoted as up to 1,080MB/s and 800MB/s for read/writes respectively, while the 256MB SM951 surpasses that by some distance with figures of 2,150MB/s reads and 1,200MB/s writes. But while this drive has humongously fast sequential speeds, it remains way off the 3.2GB/s maximum bandwidth of the 3.0 x4 bus.Surprisingly, given what Samsung is doing with 3D-NAND, the SM951 uses old-fashioned 2D planer 10nm class MLC NAND, in all probability the same 19nm 64Gb MLC NAND used in the XP941. Also added to the SM951 is support for the PCIe L1.2 power state, which can be looked at as the PCIe equivalent of DevSleep that’s supported by some standard SSDs. Another advantage of the SM951 over the previous model is that it’s bootable (with the right motherboard that is), so make sure you do your homework before buying either drive or motherboard.To test the drive at its full x4 PCIe potential, we used an Asrock Z97 Extreme6, one of the few Z97 boards that can run these drives at full speed, thanks to its additional M.2 x4 port. Alternatively, there’s Intel’s X99 chipset or, failing that, you could always use an adaptor card such as the Lycom DT-120.It takes some believing, but even the quoted figures for the sequential reads seem a little conservative when the drive was tested with the ATTO benchmark, the drive producing a score of 2,253MB/s. However, the write performance was pretty much bang on the money at 1,272MB/s. When it comes to handling the small 4K files of everyday use, the SM951 shows once again the advantage of the extra bandwidth available to it with a read speed of 44.59MB/s, in the AS SSD 4K test, compared to the XP941, but the biggest improvement comes in the write figures. The XP941 gave up a score of 73.05MB/s, which is totally eclipsed by the 132.31MB/s produced by the SM951.All these figures add up to make the SM951 an incredible SSD. In fact, the only thing that’s holding us back from going weak at the knees, is the promise of the imminent NVMe version. – Simon CrispNand type Samsung 19nm Toggle MLCQuoted sequential read speed Up to 2,150MB/sQuoted sequential write speed Up to 1,200MB/s

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