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Samsung's 840 Series SSD reviewed

Toggle DDR NAND gets a little TLC
— 10:47 AM on November 15, 2012

Samsung is the biggest producer of flash memory in the world—by a fair margin. The latest numbers from IHS iSuppli peg Samsung's share of the NAND market at over 42%, well ahead of Toshiba's at 25% and Micron's at 21%. Given those figures, it's no surprise Samsung is also one of the biggest players in the SSD business.

What may surprise you is just how long Samsung has been one of the top dogs in that realm. Everyone's familiar with the 830 Series, which debuted a little more than a year ago and quickly became one of the most desired SSDs among PC enthusiasts. The 830 Series was a follow-up to the 470 Series, which didn't make as big of a splash in hobbyist circles. Samsung had other SSDs before that, but I bet you can't name any of them.

I'm sure you can list a few major PC makers, though. Samsung claims it's been the number-one supplier of SSDs to the big-name PC brands since 2006, a full two years before Intel's first SSD even hit the market.

With the PC establishment seemingly sewn up, Samsung has increasingly targeted customers who buy drives one at a time rather than in lots of a thousand. The size of this market is growing as SSDs become more affordable for upgraders and system builders. Right now, the sweet spot is around $200, where there are numerous drives in the 240-256GB range. One of those options is Samsung's next-generation 840 Series SSD.

While the name suggests this drive is a successor to the 830 Series, the new model isn't quite a direct heir. Yes, the controller has been updated and the NAND is built using a smaller fabrication process. However, Samsung has traded the 830 Series' MLC flash for TLC chips that squeeze an extra bit into each cell. This NAND costs less per gigabyte, which is probably why the 840 Series 250GB rings in at just $180 right now. The implications of that extra bit go beyond pricing, affecting not only the drive's performance, but also its longevity. Let's take a closer look.

The skinny on TLC
TLC NAND is the defining characteristic of the Samsung 840 Series. Thankfully, I can assure you it has nothing to do with the television network responsible for spawning Here Comes Honey Boo Boo. TLC stands for "triple-level cell" and describes the number of bits (three) stored in each flash cell. The MLC NAND commonly found in consumer-grade SSDs packs two bits per cell, while the SLC flash reserved for uber-expensive server drives has only one bit.

More bits per cell translate to more gigabytes per die, which in turn means more gigabytes per wafer. MLC doubles the storage capacity of SLC, and TLC adds 50% on top of that. By adding bits, the capacity of each wafer can be increased without shrinking the fabrication process.

TLC NAND isn't a new technology; flash makers have been cranking out it for years. The triple-stuffed NAND has mostly been confined to devices like thumb drives because its endurance and performance haven't measured up to MLC NAND. For a sense of why that is, it helps to have an understanding of how data is stored in flash memory. Behold my crudely drawn flash cell diagram:

Each flash cell consists of insulated control and floating gates situated above a silicon substrate. If enough voltage is applied to the control gate, electrons will rise up from the substrate and into the floating gate through a process called tunneling. When the voltage to the control gate is cut, the oxide insulator traps the migratory electrons in the floating gate. The presence of those electrons creates a negative charge that changes the threshold voltage required to activate the cell, effectively writing data to it. Applying a sufficiently strong negative charge to the substrate reverses the process, causing electrons in the floating gate to return to the substrate. This mass exodus erases the cell and returns the threshold voltage to its lowest state.

Because there's some variation in the characteristics of individual cells, any data that's written needs to be read for verification. Data is read by asserting a voltage at the control gate and checking for current flow between the source and the drain. Current will flow if the control voltage is higher than the threshold voltage of the cell.

Reading and writing SLC NAND is fairly quick because there are only two values to consider: 0 and 1. Additional control voltages must be applied to account for the 00, 01, 10, and 11 values supported by MLC NAND, and that takes more time. The process is even longer with TLC flash, which can store eight different values between 000 and 111, requiring more control-voltage levels.

Dealing with TLC flash becomes even more challenging as the NAND starts to wear out. Electrons tunneling through the oxide layer can break down the bonds in the insulator and become trapped. The negative charge created by these stranded electrons raises the minimum threshold voltage required to activate the cell, narrowing the voltage range that can be used for programming. The more values that are crammed within that shrinking voltage range, the more difficult it is to distinguish between them. That's why TLC NAND typically tolerates fewer write-erase cycles than MLC flash, which is itself less durable than SLC NAND. Eventually, the tunneling oxide degrades to the point where the cell is no longer viable and has to be retired.

Transitioning to finer fabrication nodes reduces NAND longevity even further because the layer of tunneling oxide gets thinner as the cell geometry shrinks. That detail is particularly important for the Samsung 840 Series, whose TLC chips are fabbed on a next-gen 21-nm process. The 830 Series uses 27-nm NAND.

Should you be concerned? Maybe. Unlike some SSD makers, Samsung doesn't quote endurance ratings on its website or in the official Reviewer's Guide attached to the 840 Series. We've asked the firm on multiple occasions to characterize the drive's endurance, in terms of either the number of write-erase cycles the NAND can survive or the total volume of writes the drive can withstand as a whole, and we're still waiting for a response.

To its credit, Samsung covers the 840 Series with the same three-year warranty that applies to the 830 Series. The firm also says the NAND rolling off its production lines is sorted, and that only the highest quality chips are used in its SSDs. We've already seen Intel cherry pick high-endurance MLC NAND for enterprise-oriented drives that would have otherwise used SLC memory. Samsung appears to be doing something similar with TLC memory and consumer SSDs. Given how popular MLC-based offerings have become with the server crowd, we're not inclined to write off the 840 Series due solely to its use of TLC NAND.