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Samsung's 850 EVO solid-state drive reviewed

3D V-NAND's destiny

Samsung's 850 EVO SSD has been a long time coming. We got our first hint of its existence in July, when the firm revealed that it was developing a TLC version of its 3D V-NAND. More details on the three-bit variant were released in August, when Samsung confirmed that the flash was destined for the 850 EVO. Then, in October, mass production of TLC V-NAND chips began, setting the stage for today's formal unveiling.

The new EVO has some Shaq-sized shoes to fill. Samsung claims that its predecessor, the 840 EVO, is the best-selling SSD on the market. We haven't seen sales figures to support that assertion, but the EVO does have more than double the number of Amazon user reviews of its peers. An overwhelming majority of those—4,540 as I write this—give the drive a five-star rating.

Our expectations are especially high because the 850 EVO's elder sibling, the 850 Pro, is the fastest SATA SSD we've ever tested. Samsung's latest hotness is based on the same core technology, just with an extra bit per cell and some clever caching to pick up the slack. And it's backed by a higher endurance rating and longer warranty than most SSDs.

In the realm of SATA SSDs, the 850 EVO is kind of a big deal. Let's see what it can do.

TLC in three dimensions
The defining element of the 850 EVO is easily its three-dimensional flash. This new, three-bit implementation has the same 32-layer structure as the MLC V-NAND in the 850 Pro. The cell geometry is the same, and the die area should be similar, but the chips are fabbed at a different facility. Samsung doesn't mass-produce V-NAND and then sort the results into MLC and TLC bins; the chips in the 850 EVO were born to host three bits per cell

Indeed, this entire generation of V-NAND seems tailor-made for TLC. The individual chips offer 128Gb of capacity with three bits per cell, a much rounder number, at least in PC storage terms, than their 86Gb MLC payload. That makes perfect sense, because V-NAND's strengths nicely offset TLC's weaknesses.

Three-bit flash has lower write performance and endurance than its two-bit counterpart. The extra bit is at fault: it requires the cell to differentiate between twice as many possible values within the same limited voltage range. Writing and verifying data is more difficult—and slower—as a result. TLC's higher bit density makes the cells more sensitive to normal flash wear and more prone to interference from adjacent cells.

Source: Samsung

V-NAND tackles those weaknesses in several ways. First, it trades the floating gates typically found in planar NAND for a 3D charge-trap that's inherently more robust. This structure stores electrons in a trapping oxide that wraps around a vertical electron channel. The oxide is an insulator, so any physical damage only affects electrons in the immediate vicinity. With floating gates, a breach in the oxide can drain the entire contents of the cell, rendering it useless.

Stacking cells vertically also helps V-NAND's endurance. Instead of increasing the bit density by making the cells smaller, and thus weaker, Samsung lays down multiple layers based on a fairly large 40-nm planar geometry. The company claims this coarser 2D process puts enough distance between the cells to completely eliminate interference along the horizontal bit lines. Samsung says there's enough space between the layers to virtually eliminate interference along the vertical word lines.

With less interference to worry about, V-NAND can employ a simpler programming algorithm that improves write performance. It can also exploit the inherent robustness of the trapping oxide to program the cells more aggressively—or write less aggressively to extend the life of the flash.

TLC V-NAND still has an extra bit per cell, so it can't match the peak speed or endurance of its MLC counterpart. However, Samsung claims the EVO's flash is good enough to keep up with the planar MLC NAND found in most SSDs.

Capacity Max sequential (MB/s) Max 4KB random (IOps) Endurance
Price $/GB
Read Write Read Write
120GB 540 520/140 94k 88k/38k 75TB $99.99 $0.83
250GB 540 520/270 97k 88k/70k 75TB $149.99 $0.60
500GB 540 520/420 98k 90k/80k 150TB $269.99 $0.54
1TB 540 520/420 98k 90k/80k 150TB $499.99 $0.48

The endurance ratings certainly put the EVO in MLC territory. According to the official specs, the 120GB and 250GB versions are good for 75TB of total writes, while the 500GB and 1TB flavors can withstand 150TB. Impressively, the larger EVOs have the same endurance rating as the 850 Pro.

We don't have any V-NAND units in our ongoing endurance test, but that experiment has taught us that SSDs can last much longer than their specifications promise. Common sense also tells us that point is largely academic. PC users typically write no more than a few terabytes a year, which is much less than modern drives are rated to survive.

Note that the specs table lists two write speeds for each drive capacity. The first figure describes the performance of TurboWrite, a flash-based caching scheme inherited from the 840 EVO. TurboWrite addresses a portion of the flash in a single-bit SLC configuration that offers higher performance and endurance than even MLC setups. Incoming writes are funneled into this cache and then moved to TLC storage during idle periods. If the cache is filled before it can be flushed, inbound data proceed directly to three-bit cells, and write performance drops accordingly. The second set of write speed figures in the table above refer to TLC writes.

We're waiting on Samsung to confirm the cache sizes for the various capacities, but if the implementation is the same as in the 840 EVO, the 1TB drive has a 9GB cache, the 500GB has 6GB, and the others have 3GB. That's not loads of storage, but it should be sufficient to cover the shorter, bursty writes typical of client workloads. The 850 EVO's performance specs also suggest caching effectively offsets the performance penalty associated with lower-capacity SSDs. Despite lacking sufficient flash dies to exploit the controller's highly parallel NAND interface, the 120GB and 250GB drives nearly match the TurboWrite peaks of the higher-capacity EVOs.

In a surprise twist, the 850 EVO 1TB pictured above uses the same triple-core MEX controller as the 850 Pro, while the rest of the family taps a newer MGX chip with only two cores. Samsung contends that V-NAND's raw performance is good enough for the smaller drives to reach top speed with one fewer core. The greater "hardware automation" introduced with the MEX generation likely lessens the need for additional general-purpose horsepower, as well.

Dropping a core saves cost and power, both important factors in the solid-state sphere. SSD makers are increasingly competing on price, and Samsung is evidently shaving pennies wherever it can. Just look at the tiny circuit board used for the 250GB variant:

For additional perspective, here's how the two drives look inside their 2.5" shells:

Samsung packs a lot of other goodness into these tiny packages. The 850 EVO's 256-bit AES encryption is compliant with the TCG Opal 2.0 and IEEE 1667 standards, meeting the requirements for Microsoft's eDrive system. The drive supports the ultra-low-power DevSleep state, and it can throttle its clocks to prevent overheating in toasty environments. Then there's the five-year warranty, a premium perk that defies the series' mid-range pricing.

The EVO works with Samsung's excellent SSD Magician utility, which monitors health stats, performs firmware updates, and migrates old data to new drives. That utility also includes RAPID mode, an optional, DRAM-based caching scheme similar to Windows' SuperFetch mechanism.

We haven't tested DRAM caching in this review (there's a full suite of RAPID results in our 850 Pro coverage), but we have put the 850 EVO 250GB and 1TB through their paces against a deep field of competitors. Our performance analysis begins on the next page.