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A look at SanDisk's Extreme II SSD Architecture

By: Chris Ramseyer | Editorials in Storage | Posted: Jun 3, 2013 3:00 pm

SanDisk 19nm MLC Toggle with nCache - The Secret Weapon




We've also run across SanDisk's 19nm Toggle Mode flash before as well and not just in SanDisk branded SSD's. The 19nm flash used in Extreme II is the same flash we found in SanDisk's Ultra Plus. The flash is truly special as we found in Ultra Plus, a 4-channel SSD designed for mainstream performance. We rarely see 4-channel designs outside of the mSATA standard. The last popular 4-channel 2.5" SSD that I recall was the original Indilinx Barefoot. For SanDisk to make a 2.5" SSD that manages to outperform products like the Samsung 840 and even give LSI SandForce drives a run for the money, something has to be special.




What's special is the NAND flash and the All Bit Line (ABL) is paired with the Marvell controller in Extreme II. ABL works like double data rate (DDR) effectively, but that isn't the only thing special about Extreme II's flash.




nCache returns, another carry over from Ultra Plus. nCache is a layer of MLC flash that works in SLC mode. MLC or 2-bits per cell flash can be controlled by the controller to store just one bit, like SLC and use that faster programming to cache NAND flash writes. Since it's still NAND, it's non-volatile, meaning once the flash has your data, it's not susceptible to loss with a power failure.


Over the last few months we've spent a lot of time with both 19nm and 20nm flash. The 19nm Toshiba / SanDisk flash performs much closer to 24nm, than 20nm IMFT does 25nm. So far, the only 20nm product we've really liked was the Crucial M500 960GB. The other 20nm drives are simply slower than their 25nm versions, and the write latency is significantly higher.


This is how SanDisk explains the DRAM / Flash relationship:


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


Volatile cache - DDR DRAM cache

nCache - A non-volatile flash write cache

Mass storage - MLC NAND flash


The nCache is used to accumulate small writes (called segments) at high speed and then flush and consolidate them to larger MLC sections of the NAND Flash memory array.


SanDisk's technology innovations like ABL and nCache allow SanDisk to push forward with performance and not move in the wrong direction like we're finding in many of the new products on the market.



Final Thoughts




Throughout today, we're publishing three reviews of the SanDisk Extreme II SSD's. All three capacity sizes get apples to apples attention with comparable products of equal size and comparable price points.


Now that we have the architecture out of the way we can streamline the actual reviews and you won't need to read the same technical details over and over. Let's get started with the first review - we're starting with the SanDisk Extreme II 120GB. Find the review on our homepage or storage section archive.

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