SanDisk Extreme II SSD combines NAND storage, built-in flash cache

We've seen SSDs used as standalone drives. We've also seen them employed as solid-state caches. Now, SanDisk has combined those two roles in its Extreme II SSD. In addition to employing traditional DRAM cache memory, the Extreme II sets aside a portion of its NAND as a secondary cache. Dubbed nCache, this second-tier cache is used solely to accumulate smaller host writes before flushing them to main storage in larger chunks. Interesting.

SanDisk's own 19-nm Toggle DDR NAND lines the Extreme II, and the flash is split between SLC and MLC. The former is reserved for the nCache, while the latter is used for mass storage. SLC memory has faster write speeds than the MLC variety, so it makes sense for caching implementations. There's no need to use separate SLC modules, either. SanDisk can configure portions of its MLC NAND with one bit per cell, as it does with its single-chip iSSD devices.

Despite the Extreme II's unique caching structure, SanDisk is using off-the-shelf controller tech from Marvell: the 88SS9187. It looks like much of the magic happens in the firmware—and in the NAND.

According to SanDisk, the NAND is rated for 3,000 write-erase cycles. That figure likely refers to the MLC portion of the flash; SLC memory typically has greater endurance because NAND degradation has less of an impact when there's only one bit per cell. In any case, there appears to be little need to worry about endurance. SanDisk claims the Extreme II can endure at least 80TB of writes over its lifespan. The drive is also covered by a five-year warranty.

As I type this, a SanDisk Extreme II SSD is making its way to our labs. We should have a closer look at the drive soon. In the meantime, you can chew on these stats: SanDisk says its latest SSD pushes 550/510MB/s with sequential reads/writes and 95k/78k IOps with random I/O. Those performance ratings put the Extreme II in the upper echelon of consumer-grade SSDs, and we're eager to see if it can keep up with the competition in our tests. Stay tuned.

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