Samsung SM951 128GB And 256GB SSD Review

Samsung's SM951 is the hottest storage product on the market. Today we look at the two smaller versions, both of which still perform better than SATA.


The SM951 story has developed since we first shucked a drive from a Lenovo X1 Carbon Gen 3 two months ago. Resellers are selling them at better-than-expected prices. Samsung also announced a new NVMe model appropriately called the SM951-NVMe. The company's 512GB SM951 offers attractive performance for its price, but many waiting for an even better value want information on the smaller capacities before jumping into the world of PCIe-based SSD performance.

If you read our first SM951 review, then you already know that the drive we pulled from Lenovo's Ultrabook failed to reach Samsung's SM951 performance specification for sequential reads. At the time, we suspected that Lenovo was limiting the drive to conserve battery power. With more samples in our hands, we now know the issue is Lenovo-specific, and it doesn't drives sold by other vendors.

Today we're testing the 128 and 256GB versions of Samsung's SM951 next to the XP941, which officially surfaced in 2014. Back then, the XP941 was the only native PCIe SSD on the market. Marvell has since joined the fight with two native PCIe controllers that will also be represented in the benchmark charts today.

Technical Specifications

The three SM951 drives we're testing use AHCI, and are not the rare NVMe interface. In the coming weeks, NVMe-based models will begin surfacing with channel availability soon after. Several online shops already list pre-order status for the NVMe versions, though we've yet to hear of anyone with product on-hand.

The 512GB SM951 we have is Lenovo-specific, with the part number MZHPV512HDGL-000L1. That L1 at the end is a reference to Lenovo. The last five digits indicate the region the drive is sold in. Common codes there are 00001 to 00004, with HP and Lenovo assigned special modifiers. We can also assume that Dell has its own code, since the company plans to release Ultrabook models with SM951 SSDs. 

We already know that Lenovo's SM951 has limited sequential read performance. In our testing, we achieved roughly 1700 MB/s, while the 256GB SM951 we received from RamCity is capable of the full 2150 MB/s. The RamCity 128GB sample achieved 2050 MB/s. Because of this, we've noted the Lenovo model in our charts as a Lenovo SM951 512GB. Soon, we'll have a fresh 512GB drive to test for an upcoming editorial that covers the best PCIe and SATA 6Gb/s SSDs, as well as the fastest hard drives on the market today.

The 512GB and 256GB SM951 models share nearly identical performance specifications. The 256GB model loses some write performance. And the 128GB version drops to "just" 600 MB/s sequential reads. At least it retains the same 70,000 random read IOPS rating as the other models.

Pricing And Accessories

The SM951s come from Samsung's SSI group, which sells to OEM customers. These products are normally not available to end-users. But demand for the company's SSDs compelled Samsung to let a limited supply bleed through to a few resellers large enough to have accounts with key distributors. Vendors spec their own warranty terms, conditions, pricing and even packaging.

We get all of our SSI products from RamCity, an Australian-based company. RamCity sells direct and also has an Amazon account where products are stored and shipped from North America. The SM951s are hot items, so prices are subject to change based on availability. The Australian site advertises in Australian Dollars with Australian VAT taxes that do not apply to orders coming from other countries.

A Closer Look At The SM951 SSDs

Both of the drives we're testing today share the same layout, controller and DRAM. The main difference between them is that the number of dies per package doubles on the 256GB SM951.

All Samsung SM951 SSDs ship in the M.2 2280 form factor. The firmware installed on both of our samples is BXW25000. The Lenovo model's firmware is BXW22L00.

The two lower-capacity SM951s use just two NAND flash packages.

The smaller drives are also single-sided; all of their surface-mount components are on one side. The 512GB model is double-sided. This can be important, since some companies use a single-sided connector that sits closer to the motherboard.

All three capacities employ the same Samsung UBX controller. 

The two lower-capacity models use the same DRAM, while the 512GB model ships with twice as much DRAM to cache page table data. 

The flash is the same 1xnm across all three models. Only the number of dies per package increases alongside capacity. The two drives we're testing today sport two packages. We suspect Samsung uses quad-plane flash, and that is how it manages to reach this level of performance.

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Sequential Read

To read about our storage tests in-depth, please check out How We Test HDDs And SSDs. Four-corner testing is covered on page six.

Right from the start, both of the SM951s from RamCity outperform the SM951 from Lenovo. Normally we'd expect the higher-capacity SSD to perform better thanks to its increased parallelism. But in this case, the difference between the 256GB and 512GB models is large enough to fit a SATA 6Gb/s SSD in. You could park the peak performance of an 850 Pro in the gap!

It appears that Lenovo limits the SM951 for its Ultrabooks. This would reduce power consumption and the amount of heat generated inside its compact chassis.

Sequential Write

The difference in sequential write performance between the three SM951s is significant. That wave in the 512GB model's output could be due to thermal throttling. We ran the test several times and came away with the same pattern. The SM951's controller runs cooler than the XP941's. As a major manufacturer, Lenovo could have asked Samsung to build a drive with a lower TDP for its Ultrabooks.

Random Read

Random performance ties in with latency, what makes your computer feel fast or not. It's hard to believe, but the latest M.2-attached SSDs exceed 12,000 random read IOPS at a queue depth of one. On the line chart, we see that all three SM951s outperform Samsung's specifications. And again, the two lower-capacity models deliver better performance than Lenovo's.

Random Write

At low queue depths, all three SM951 drives are in a league of their own. Removing the Lenovo SM951 from the group, the 256GB implementation delivers nearly 10,000 more random write IOPS at a queue depth of one. Samsung is one of the few SSD manufacturers that focuses on low queue depth performance. Samsung is also one of the only companies that tells you what random read and write performance at a queue depth of one to expect.

Mixed Workloads

Our mixed workload testing is described in detail here, and our steady state tests are described here.

Mixed workload performance in client workloads has always been an important consideration. But until recently, it was largely ignored since SATA-based drives can only transmit or receive data at one time. PCIe is bi-directional, but the flash controller also needs to take advantage of two-way communication.

Samsung's XP941 delivers around the same mixed workload performance as the best SATA products. When I first tested it, I introduced our mixed workload testing on client SSDs. Since then, Samsung has worked on improving those results. The SM951 is optimized for sending and receiving data simultaneously, outpacing other M.2-based drives. This is an area where we expect the new SM951-NVMe to really stand out once it becomes available.

128KB Sequential Mixed-Workload Steady State Performance

For this review and for future stories with a workstation twist, we're adding a look at 70%-read steady state mixed sequential transfers.

The line chart shows sequential steady state performance from 100% reads to 100% writes at high queue depths. The two larger SM951s perform well beyond competing products. The 128GB model falls in line with other products on the market at when the mix moves closer to a static workload. 

4K Random Write Steady State Performance

As in the previous test, Samsung's 128GB SM951 gets lost in the mix as the higher-capacity models shoot above the crowd. Kingston's HyperX Predator performs well here thanks to Marvell's new Altaplus controller. Still, none of the drives on the chart hold a consistent level like Intel's SSD 750.

PCMark 8 Real-World Software Performance

For details on our real-world software performance testing, please click here.

After watching the SM951 dominate our synthetic tests, we aren't surprised to see it at the top of our real-world benchmarks as well. The gathered from PCIe-based M.2-attached drives are even tighter than what we gathered from SATA-based drives. This really goes to show that if you are not multitasking or writing enough data to induce steady state conditions, you won't notice a lot of difference between this class of SSDs.

Total Storage Bandwidth

The slight performance differences compound over time. Here we see the converged results, and the SM951s are at the top. But that isn't a surprise. Meanwhile, the XP941s fall back a bit, mixed in with the Marvell-based drives. Samsung appears to be a full generation ahead of Marvell in performance.

PCMark 8 Advanced Workload Performance

To learn how we test advanced workload performance, please click here.

Even though the Lenovo-specific SM951 demonstrates lower read performance than the two lower-capacity models, higher write speed is achieved through parallelization, keeping the 512GB implementation at the top of our chart under heavy and light workloads. The 256GB SM951 falls in line just behind, while the 128GB drive with the fewest number of dies blends in with higher-capacity models using Marvell controllers.

Latency Tests

The two larger SM951s yield the best throughput of the drives on our chart, but the higher-capacity XP941s complete the tasks in less time. The Lenovo-specific SM951 lands just behind last year's OEM superstar from Samsung. Meanwhile, the smaller SM951s take longer to finish the tasks under light workload conditions coming out of heavy workloads.

Lower-capacity Samsung SSDs are hit and miss when it comes to cleaning up the NAND after heavy workloads. Many of the company's products displayed this behavior. What we're seeing is the clean-up process taking longer. New reads and writes happening at the same time as background operations increases latency. I noticed this with the 830- and 840-series drives. The 850s with 3D V-NAND were more resilient, even after especially taxing workloads. The SM951 uses 1xnm 2D planar MLC NAND that evolved from what we found in the 840-series, though.

The light workload set assumes that five minutes between tests is long enough for the drive to tidy up dirty cells. So either it takes longer to clean house, or the house gets dirty faster with the large 128Gb dies. Both the 128GB and 256GB SM951s use just two NAND packages and the 512GB model uses four. This can limit I/O, even with quad-plane packages.

Here we see the 128GB SM951 in a dirty state after a reasonable number of sequential and random writes. At this point in the test process, the drive wasn't being pushed particularly hard. Despite that, 128KB sequential write performance is already down to very low levels. Over time, the internal wear-leveling algorithms will clean the flash and write performance should recover where cells are free.

Notebook Battery Life

For more information on how we test notebook battery life, click here.

After our first SM951 test, Lenovo released a firmware update that lets us adjust the amount of time it takes to drop into slumber. We updated the firmware and set this timer to two minutes. Then, we retested all of the drives for this chart.

Samsung's SM951 is the first client SSD with support for the L1.2 power state defined by PCI-SIG. At full rest, it draws just 2mW. What really surprises us is just how much better Samsung's drives do in this discipline than other SSDs with Marvell controllers.

On battery power, your notebook scales back several system bus clock speeds to save power. In this reduced power state, all of the drives in today's chart perform nearly identically. We sometimes find a product that stands out from the rest, providing better efficiency. For the time being, though, one M.2 PCIe-based SSD fares just as well as the rest.

Final Thoughts

Samsung enjoys a large lead in both controller and flash technology over its competitors. 3D V-NAND stacked 32 layers-high covers the flash. Only Samsung has this. And the UBX isn't Samsung's first PCIe-to-flash controller efficient enough for power-restricted M.2 applications; it's a second-generation part.

Marvell is trying to catch up, but even the second-gen Altaplus controller trails Samsung's effort. SandForce, now part of Seagate, could make some noise at Computex. However, the SF3700 is two years late and we haven't seen anywhere near 2150 MB/s in our behind-the-scenes testing. Phison, JMicron and others plan to release PCIe-based controllers in 2015. We're not holding our breath for Samsung-class performance right out of the gate, though.

Our story today focused on the M.2 form factor, so Intel's SSD 750 wasn't represented. In a few days, an SSD 750 400GB will arrive. Right behind that we should be receiving a trio of SM951-NVMe drives. Once all of the pieces are in place, we'll publish a comparison of high-end NVMe products.

I hate to say that the products in this piece are in a lower-tier, but the SM951-NVMe is going to be more expensive. And we don't expect a large performance increase over the AHCI-based SM951 in client workloads, so the models we tested should provide better value to enthusiasts. Without samples in the lab, we can't say for sure. At some point, however, excess becomes superfluous. Time will tell.

The 128 and 256GB SM951s raise some questions about the 512GB model's performance. The Lenovo-specific drive is certainly fast, though there are areas we'd expect it to be quicker if there were no restrictions on its speed. The difference in sequential reads between all three models at a queue depth of four is 500 MB/s, or basically a SATA 6Gb/s SSD. 

Samsung's 128GB SM951 is a great value for non-gamers. Games take up a lot of space, and some of us don't get to play as often as we'd like. Others just want a notebook capable of blazing-fast performance. Business users can certainly appreciate the extra battery life, and excellent 4KB read performance makes quick work of large email databases.

The 256GB model gives you room to store a few of your favorite games and professional applications. It delivers 90% of the performance at nearly half the cost of the 512GB drive. That wouldn't mean much in the congested SATA space. But this is PCIe-attached. It's the future, where you'll find the real performance story. Consider 256GB today's sweet spot.

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Chris Ramseyer is a Contributing Editor for Tom's Hardware, covering Storage. Follow him on Twitter and on Facebook.

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