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SUPPLEMENT SSD


Engineering MLC Flash-Based SSDs to Reduce Total Cost of Ownership


in Enterprise SSD Deployments This article highlights how STEC’s CellCare technology enables the use of MLC flash-based SSDs to reduce total cost of ownership (TCO) while meeting the stringent requirements of next generation enterprise storage applications.


T


he introduction of higher density, lower cost MultiLevel Cell (MLC) flash components in consumer electronics products has driven dramatic growth in these markets. To bring the high-performance and low power of MLC-based solid-state drives (SSDs) to enterprise-class server and storage applications requires higher-density SSDs at more competitive prices.


While cost reductions triggered by MLC flash have set the stage for wider SSD adoption in the enterprise, the industry requires design techniques to overcome the inherent deficiencies of MLC flash technology. MLC flash is attractive for its price and density characteristics, but its relatively poor endurance and performance characteristics (in its raw state) make it a challenge for enterprise applications. The reliability and endurance characteristics of MLC SSDs must be improved to extend them into enterprise-class applications that demand the ultimate in performance and reliability. Without these improvements, the adoption of MLC flash- based SSDs would be limited to consumer applications.


Simply substituting MLC flash media for Single Level Cell (SLC) flash will not by itself enable the lower cost and improved reliability and endurance that SSDs require for the enterprise. To meet these more stringent requirements, MLC flash-based SSDs must be designed specifically for enterprise environments. As a leading global provider of solid-state technologies and products, STEC’s CellCare™ technology meets this set of requirements for enterprise-class endurance and performance through the use of advanced signal processing and adaptive flash management algorithms.


NAND Flash Memory Types To understand the benefits of CellCare technology, we need to review the different types of flash memory available. The three types of NAND flash memory include SLC, MLC and ‘enhanced’ MLC (eMLC). The main difference between SLC and MLC/eMLC is the number of bits stored per NAND cell. SLC stores 1-bit of data per cell, while MLC and eMLC store 2 bits. The main differences between these flash memory types is captured in Table 1.1


SLC NAND flash typically supports more than 100,000 write cycles per cell and provides a very reliable and useful life of 10 to 20 years, making it the flash technology of choice for enterprise applications. MLC flash memory is less reliable and withstands no more than 3,000 write cycles per cell in 3x nanometer process geometries. The limited write endurance of MLC flash makes it well suited for consumer applications,


where the media is used primarily for reads. To attain marginally higher endurance, eMLC flash memory uses a die- screening process of consumer MLC flash or a static tweaking process of flash process parameters, such as increasing erase time (to slow down the wear-out characteristics) and program time. The result is higher endurance, but with slower performance, rendering eMLC flash unsuitable for true enterprise-class applications.


1 MLC page size is 2x SLC page size. tPROG cycle for MLC enables 2x writes compared to SLC.


SLC, MLC and eMLC Comparison 3xnm Lithography Bit/Cell


Endurance


ECC Capability Density tPROG tERASE


Useful Life Performance 100K


8b/512B 16GB 0.5ms


1.5-2ms Constant


SLC Flash MLC Flash eMLC Flash 1


2 3K


24b/1KB 32GB 1.2ms 3ms


Reduces Table 1: SLC, MLC and eMLC Flash Memory Comparison


At present, neither MLC nor eMLC flash meet the endurance requirements of 10 full drive capacity writes-per-day, and as process geometries shrink, the endurance and retention capabilities will significantly decrease as well. Additionally, the write performance of MLC flash is 30% to 40% slower than SLC flash media, and eMLC flash performance is 20% slower than MLC flash. Both MLC and eMLC flash require higher error correction code (ECC) capabilities to match those of SLC flash.


Additionally, the bit error rate (BER) of eMLC and MLC flash increases during the useful life of the product which makes the components more prone to failure as they are programmed and erased. The read performance of eMLC flash deteriorates during the life of the product, which makes eMLC flash unsuitable for ever-intensive enterprise


October/November 2011 I www.snseurope.info SSD9 2


10K-30K 24b/1KB 32GB


2-2.5ms 3-5ms


Reduces


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