The rest of the 840 Series SSD
There's more to Samsung's 840 Series SSD than its 21-nm TLC NAND. Heck, there's more to the NAND than the fab process and the number of bits per cell. Like the last two generations of Samsung SSDs, the 840 Series' flash memory conforms to the Toggle DDR NAND specification. This standard was jointly developed with Toshiba and is an alternative to the ONFI flash spec backed by Intel, Micron, and Hynix.
Toggle DDR is capable of executing reads and writes on both the rising and falling edges of a data strobe, hence the DDR moniker. Synchronous ONFI NAND has a similar double data rate driven by an external clock cycle. That external clock is always running, while Toggle DDR turns on its data strobe only when transfers are taking place. As a result, Toggle DDR chips should be more power-efficient at idle than their ONFI counterparts. We'll put that notion to the test a little later in the review when we probe the 840 Series' power consumption.
Although Samsung has equipped its SSDs with Toggle DDR NAND for a couple of years, the 840 Series is the first to use flash chips compliant with version 2.0 of the Toggle standard. The initial spec allowed for transfer rates up to 133 MT/s, a ceiling that's been bumped up to 400 MT/s for Toggle DDR2. That 400 MT/s limit nicely matches the maximum data rate of the ONFI 3.0 specification, by the way.
To support the 840 Series' faster NAND, Samsung has cooked up a new revision of its proprietary, triple-core SSD controller. Like the 830 Series' MCX controller, this new MDX chip has eight memory channels and ARM9-based processor cores. Multiple cores are used to prevent background tasks like garbage collection from slowing the drive's performance with user workloads. The firmware controls which tasks are assigned to the individual cores, and there's some flexibility to shuffle the load around.
Samsung always makes a point of highlighting the fact that its controller technology doesn't use SandForce-style write compression. A little compression mojo might not be a bad idea given the inherent endurance handicap associated with 21-nm TLC NAND, though. Compression allows fewer blocks to be written to the NAND, reducing the write amplification factor and leaving more write-erase cycles in reserve. This approach biases performance toward easily compressible data, but there are ways to reduce the write amplification factor without resorting to compression. SSDs can also wring more life out of their NAND with smarter error correction and signal processing algorithms. Alas, we couldn't pry any specifics from Samsung regarding its use of those techniques.
We do, however, know that the 840 Series attempts to extend its lifespan by reserving more NAND capacity as spare area available only to the controller. The drive's 120, 250, and 500GB capacities are a bit lower than the 128, 256, and 512GB you might recall from the 830 Series.
Lower capacities usually hint at the presence of RAID-like schemes that protect against physical flash failures. The 840 Series doesn't arrange its NAND in a redundant array, though. Samsung restricts NAND-level redundancy to its enterprise-grade drives. It does, however, endow the 840 Series with full-disk encryption support.
Despite the fact that the 840 Series is aimed at the mainstream market, the family doesn't include anything below 120GB. Perhaps that's because the base model's street price is already quite low, at just $100.
If you think that's low, check out the sequential write speed ratings for the various members of the 840 Series lineup.
|Capacity||Max sequential (MB/s)||4KB random (IOps)||Price|
Ouch. The 250GB model's 240MB/s maximum sequential write speed doesn't inspire confidence. To put it in perspective, consider that the old Samsung 830 Series 256GB is rated for 400MB/s sequential writes. That drive has the same write speed rating as its 512GB sibling, but there's a sizable gap between the 250 and 500GB versions of the 840 Series. I suspect the larger 840 Series drive uses a greater number of NAND dies, allowing it to exploit more parallelism within the controller. We've asked Samsung to reveal the size and number of NAND dies for each member of the 840 Series but are still awaiting an answer.
For what it's worth, the performance of contemporary MLC SSDs tends to plateau at around 256GB. TLC NAND can serve the same capacity with fewer dies, but it looks like the 840 Series 250GB doesn't have enough dies to take full advantage of the controller.
Of course, the NAND dies aren't the only memory chips onboard the 840 Series. The drive features DRAM cache memory used primarily to store lookup tables. This low-power DDR2 cache also serves as a landing pad for some incoming host writes. The 120GB model has a 256MB cache, while the higher-capacity flavors sport 512MB.
We'd love to show you a gratuitous close-up of the 840 Series' DRAM cache and other chips, but Samsung has locked down the case with pentalobe screws straight out of Apple's playbook. (We're in the process of obtaining the tool required to crack open the case.) In some ways, the drive bears an eerie resemblance to the iPhone 5. The case is black; it's rectangular with rounded corners; and a chamfered edge runs around the rim. At just 7 mm thick, the 840 Series should be slim enough for Apple, too—and for folks looking to upgrade thinner notebooks that can't accommodate the 9.5-mm cases that house most SSDs.
Speaking of upgrades, the SSD Magician utility that accompanies Samsung SSDs is one of the best around. There's secure-erase functionality, of course, plus optimization routines and a built-in benchmark. The integrated flashing utility can reach out and grab updates directly from Samsung, and the overprovisioning can be tweaked to allocate more NAND capacity as spare area. Don't get too excited about that cloning icon, though. It refers to a copy of Norton Ghost 15 that's included only with drives sold as part of upgrade kits.
If you want to keep tabs on the health of the 840 Series' TLC NAND, drilling down in the System Information section of the interface reveals an estimated lifetime display based on SMART data. After hammering our drive with several days of testing, the needle remains close to 100%. Speaking of testing, it's time to see how this puppy performs.
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