SanDisk's Extreme II SSD Uses "Two Tier Caching" to Improve Performance

Paul Lilly

Sequel is faster than the original.

SanDisk today unveiled its Extreme II SSD series, a follow-up to the original Extreme SSD that we reviewed last year (we evaluated the 240GB model). The Extreme II SSD line is supposedly faster than the original in most instances, part of which is due to the use of a select amount of single-level cell (SLC) NAND flash memory for what SanDisk describes as a "two tier caching" setup.

"The SanDisk Extreme II SSD supports a unique feature to improve random write performance and ensure a very positive user experience," SanDisk explains. "Studies show that modern operating systems mostly access the storage device using small access blocks, with the majority being 4KB access blocks. The small logical access blocks conflict with the physical block structure (>1MB) for the nwer generation flash memory technology. Therefore, to bridge this different SanDisk SSDs employ three storage layers."

The first of those layers is volatile cache, or DDR DRAM cache. Next up is nCache, a non-volatile flash write cache, which is where the SLC NAND flash memory comes into play. And the third is mass storage, your typical MLC NAND flash memory chips. SanDisk says the nCache (SLC NAND flash) is used to acculmulate small writes, called segments, at high speed and then flush and consolidate them to larger MLC sections.

SanDisk isn't saying how much SLC NAND flash memory it's equipping its drives with, though the end result is faster read and write speeds in most real-world and synthetic applications. Furthermore, SanDisk claims its Extreme II SSDs are the highest performing retail SSDs to date.

The new drives will be available in 120GB ($130), 240GB ($230), and 480GB ($440) capacities.

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