Samsung SM951-NVMe (256GB) PCIe SSD Reviewby Kristian Vättö on June 25, 2015 9:40 AM EST
PCIe and especially NVMe SSDs are without a doubt the hot topic in the SSD industry at the moment. There are still only a handful of drives on the retail market, but as we saw at Computex a few weeks ago, everyone is working closely on their PCIe designs and we should see more entries to the market later this year with a big wave of PCIe SSDs arriving in the first half of 2016.
Samsung has always been an early adopter in the SSD space. The company was the first one on the market with a PCIe 2.0 x4 M.2 SSD the (XP941) back in late 2013, and before that it was the first one to adopt TLC NAND in 2012. Earlier this year Samsung's second generation client PCIe drive, the SM951, made an appearance in a Lenovo laptop, but to everyone's surprise the drive wasn't NVMe compatible like Samsung had announced earlier. After discussing with Samsung, the company said they it has an NVMe client drive in development, but it declined to provide any reasoning as to why the SM951 still used the AHCI driver stack.
To our surprise, Ganesh found an NVMe-enabled Samsung M.2 SSD inside Intel's Broadwell-U NUC a while back. This was rather confusing at first because Samsung had specifically told us that the SM951 doesn't support NVMe, but after a closer look and a series of emails with Samsung the drive turned out to be an NVMe version of the SM951, or SM951-NVMe as Samsung calls it.
Distinguishing the AHCI and NVMe version from each other isn't very simple as the difference lies in a single character in the model number. The AHCI version carries the code MZ-HPVxxx0 (where xxx is the capacity in gigabytes), whereas the NVMe version is called MZ-VPVxxx0. Since both versions of the SM951 are technically OEM-only, the close naming isn't really an issue, but if you are shopping for the SM951 I recommend that you take a close look at the part number before pulling the trigger to ensure that you get the version you are looking for.
SM951-NVMe on the left, SM951-AHCI on the right
On the hardware side the AHCI and NVMe versions of the SM951 are a match. Both utilize Samsung's S4LN058A01-8030 controller dubbed as UBX, which is a PCIe 3.0 x4 controller that apparently supports both AHCI and NVMe driver stacks. That isn't surprising, though, because nearly all client-grade NVMe controllers I know are capable of supporting both -- it's just a matter of developing two separate firmware builds. The firmware development is likely the reason why the SM951-AHCI was the first one to market because Samsung already had the basic AHCI firmware from its XP941 and SATA drives, whereas the SM951-NVMe needed more development from scratch given how different and more efficient the NVMe command set is.
|Samsung SM951 NVMe Specifications|
|Form Factor||M.2 2280|
|Controller||Samsung S4LN058A01 (PCIe 3.0 x4 NVMe)|
|NAND||Samsung 16nm 64Gbit MLC|
|4KB Random Read||300K IOPS||300K IOPS||300K IOPS|
|4KB Random Write||83K IOPS||100K IOPS||100K IOPS|
Similar to the AHCI version, the SM951-NVMe only comes in capacities of up to 512GB. The reason lies in NAND because the SM951 utilizes 64Gbit dies, and with only four NAND package placements on the M.2 2280 PCB the maximum capacity with 16 dies per package works out to be 512GB (8GB x 16 x 4). It seems that Samsung doesn't have a high volume 128Gbit MLC die at this point, although we will likely see one with third generation V-NAND later this year. The first generation V-NAND die is 128Gbit, but since it only has 24 layers it's not cost efficient for a client drive, especially not for an OEM-specific one given how cost sensitive PC OEMs are.
TechInsights found out that the NAND in the SM951 (both AHCI and NVMe) is actually 16nm. While I was aware of the change in generation character of the part number, I believed that it would just be a second generation of Samsung's 19nm die because to me it didn't make any sense that Samsung would build a 16nm 64Gbit die. I'm working on an article comparing all the modern 15-16nm NAND processes, so stay tuned for more in-depth analysis of Samsung's 16nm node.
One of the major questions with every PCIe SSD is whether it is bootable. Back when the XP941 became available the situation was rather messy because motherboard OEMs had not prepared for PCIe SSDs yet, which require BIOS/UEFI support from their side in order to show up in the boot menu. Fortunately, most OEMs fixed this for 9-series motherboards and now most models have a BIOS update available with proper support for PCIe and NVMe SSDs.
In short, the SM951 NVMe is bootable in my ASUS Z97 Deluxe with the latest 2401 BIOS. I don't have any other 9-series motherboards at hand, but I suspect that any motherboard with advertised NVMe support and appropriate BIOS will boot from the SM951 NVMe.
For tower Mac Pro users the story isn't as pleasant, though. I put the SM951 NVMe inside my 2009 Mac Pro, but OS X wouldn't even recognize the drive. Despite the fact that the custom Apple SSD inside the MacBook is NVMe based, I suspect that the current version of OS X doesn't carry a general NVMe driver, and even if it did the Mac Pro and its chipset might simply be too old to support NVMe, which honestly isn't surprising for a +5-year-old system. In any case, Mac Pro users can still buy and boot from the AHCI version of SM951, but I wouldn't hold my breath for any NVMe support in the future.
The SM951-NVMe is an OEM part, meaning that availability is very restricted. The drive is listed by a handful of online retailers, but none of them seem to have it in stock yet. RamCity is expecting stock in mid-July, but told us that even that is uncertain because its distributors are still saying that the NVMe version is in sampling stage with no schedule for high volume availability. We got our 256GB sample directly from Samsung, hence the early access, as it seems that there is no way to buy the SM951-NVMe at this point. I will provide an update when I hear more about the availability.
|AnandTech 2015 SSD Test System|
|CPU||Intel Core i7-4770K running at 3.5GHz (Turbo & EIST enabled, C-states disabled)|
|Motherboard||ASUS Z97 Deluxe (BIOS 2205)|
|Chipset Drivers||Intel 10.0.24+ Intel RST 126.96.36.1990|
|Memory||Corsair Vengeance DDR3-1866 2x8GB (9-10-9-27 2T)|
|Graphics||Intel HD Graphics 4600|
|Desktop Resolution||1920 x 1080|
|OS||Windows 8.1 x64|
- Thanks to Intel for the Core i7-4770K CPU
- Thanks to ASUS for the Z97 Deluxe motherboard
- Thanks to Corsair for the Vengeance 16GB DDR3-1866 DRAM kit, RM750 power supply, Hydro H60 CPU cooler and Carbide 330R case
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kspirit - Saturday, June 27, 2015 - linkThank you, I appreciate the response. I'll check it out :)
fastfood8891 - Thursday, June 25, 2015 - link@Kristian Vättö
For that one graph on page 3, why do you divide IOPS by the standard deviation? I understand you are showing that this drive is a lot more consistent than the others, I just wonder what the mathematical intuition behind this value is.
Kristian Vättö - Thursday, June 25, 2015 - linkIt's just a key ratio. Standard deviation alone is a bad metric because it doesn't take the performance into account at all. It wouldn't be fair to compare stdev of a drive that does 15K IOPS against a drive that does 5K IOPS because if the two had equivalent stdev the impact would be much more severe on the slower one (e.g. frequent 50% drops in performance, whereas only 15% for the 15K IOPS drive).
krumme - Friday, June 26, 2015 - linkEnjoy this man guys while we have him. This is competence at its finest.
JellyRoll - Friday, July 10, 2015 - linkWell, if competency consists of him originally presenting test results wrong in several articles, and then a reader correcting him in the comments of the 750 article, and then him adopting the correct method...well, yeah. Then that is competency. (that is what happened)
fastfood8891 - Saturday, June 27, 2015 - linkmakes sense, thank you for the explanation.
Samus - Thursday, June 25, 2015 - linkFirst gen. Give it time. Seems like a dedicated nvme controller (ala Intel) would improve random performance...it looks like this controller is already running near its capability in AHCI mode.
But as with everything nand, mature firmware will help
willis936 - Thursday, June 25, 2015 - linkExciting stuff. Does anyone know if current broadwell laptops can be retrofitted with nvme M.2 drives in the future? Specifically the xps 13 2015. It's a hot item with poor storage options.
extide - Thursday, June 25, 2015 - linkThat laptop, probably yes. Best place to check would be the notebookreview forums -- they should have a specific forum for that laptop and you can ask there, or see if someone has already swapped in a PCIe drive. -- You need to verify 1) That the m.2 slot is actually pcie and not just sata, and 2) that it supports nvme in the bios, but being broadwell, you can probably assume it does.
lilmoe - Thursday, June 25, 2015 - linkIs there any tangible difference in CPU utilization using NVMe? Would be nice if there was a graph...