Clever caching fuels Samsung 840 EVO SSD

Samsung just announced its 840 EVO solid-state drive. The initial press release covered the basics, but it was a little short on the nerdy details we tend to obsess over here at TR. We've since learned much more about the drive, and we'll have a full review soon. In the meantime, we can share a few interesting tidbits about the EVO and its unique approach to caching.

Improved write performance is one of the EVO's defining characteristics. The boost in write speeds is made possible by a feature called TurboWrite technology, which is essentially a high-speed write cache built into the NAND. Most of the EVO's flash memory is configured as TLC NAND with three bits per cell. However, the portion dedicated to TurboWrite is addressed as one-bit SLC NAND. The single-bit flash offers higher write performance than TLC memory, but it has only one third the storage capacity per cell.

TurboWrite sounds similar to the nCache scheme employed by SanDisk's Extreme II SSD, but there are a few key differences. The Samsung implementation is a pure write buffer; it caches all incoming host writes regardless of whether they're sequential or random. Cached writes are only moved to main storage during idle time, though host writes may be sent directly to main storage if the TurboWrite buffer is full. The size of the write cache varies depending on the drive's capacity. The 120 and 250GB models can cache up to 3GB of data, while the higher-capacity models have even larger buffers.

The 840 EVO has a second layer of caching enabled by Samsung's SSD Magician software. Dubbed RAPID mode, this scheme uses system DRAM to accelerate system performance. When enabled, RAPID mode allocates up to 1GB of system memory to storing frequently accessed data. This software cache is also used to "optimize" host writes before they hit the SSD. Samsung showed RAPID mode dramatically improving the 840 EVO's read and write performance in a couple of synthetic benchmarks, and we're eager to try the feature ourselves.

RAPID mode isn't just for the 840 EVO. The feature is also coming to the existing Samsung 840 Pro. You'll need to be running Windows 7 or Win8 to use the feature with either drive.

The 840 EVO is slated to replace the standard 840 Series, and it will be priced accordingly. Expect to pay $110, $190, $370, $530, and $650 for the 120, 250, 500, 750, and 1000GB models, respectively. Like its predecessor, the 840 EVO has a three-year warranty.

I'm already late for the next event on the agenda at Samsung's Global SSD Summit, so I don't have time to discuss the EVO further. I can, however, provide a couple of morsels about the NAND. The 128Gb chips are fabbed on a 19-nm process, which means they're likely larger than the 16-nm flash Micron announced earlier this week.

Samsung hasn't released an official endurance specification for the chips, but it says they're surviving about 3,700 write-erase cycles under current testing. That sounds pretty high for TLC NAND fabbed on a sub-20-nm process. Samsung makes a lot of NAND, though, and it selects only the best chips for use in its SSDs. This SSD-grade flash is claimed to have 20X fewer bad blocks than the firm's "normal" flash.

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