Choosing the Right SSD for the Enterprise
Most SSDs today use NAND-based flash memory, which retains memory even without power. The type of NAND used has historically defined its endurance. NAND-based flash memory comes in two types: single-level cell (SLC) NAND and multi-level cell (MLC) NAND. MLC has been used primarily in consumer devices while SLC is used in the enterprise for mission -critical applications due to tradeoffs between endurance and cost.
One characteristic of all NAND flash is that once data has been written and then deleted from a NAND flash block, that data must be erased before any new data can be written. This program/erase process eventually breaks down the oxide layer that traps the electrons and the NAND flash wears out. With SLC, one bit of data occupies one cell of flash memory. SLC is able to withstand roughly 100,000 Program/Erase cycles or as much as 10x the number of cycles that can be handled by MLC NAND. Thus, SLC NAND offers far greater endurance. The downside is that SLC is more expensive and delivers less capacity than MLC. SLC, therefore, has been used in enterprise and niche applications where budget was not a primary concern.
MLC comes in 2-bit-per-cell and 3-bit-per-cell varieties that hold larger capacities than SLC. However, 3 bit-per-cell MLC can withstand only about 10,000 P/E cycles and 2 bit-per-cell MLC 3-5,000 P/E cycles, making MLC less reliable, over time, than SLC. Additionally, because the consumer market that currently drives NAND manufacturing depends on MLC technology for cameras, video recorders, USB sticks and other consumer electronics, MLC volumes are considerably higher than SLC and the cost of manufacturing MLC is significantly lower.
MLC vs SLC Technology
Adapting MLC to the Enterprise
About three years ago, believing that high prices of SSDs were limiting sales to budget-constrained enterprises, SSD vendors began offering the lower cost MLC for the enterprise. Today, enterprise MLC is about one third the price of SLC .
One way vendors have adapted MLC is by creating so-called enterprise MLC (eMLC). In modern, high-density MLC Flash manufacturing, the data reliability of a flash die can vary considerably even across the same wafer. By identifying premium die, manufacturers can provide casts with the highest data reliability and longest life. These choice die are trimmed and subject to server component flow test cycles that are more rigorous than the standard MLC test processes. Die that pass this test are called eMLC NAND flash. These eMLC typically can sustain about 3x the P/E cycles (30,000) of standard MLC, producing higher reliability SSDs for write-intensive applications.
SSD manufacturers are also improving their SSD controllers, the command center for NAND memory that determines where each memory cell will read or write data and communicate with the computer interface. The way the controller responds and works to correct errors is a critical part of their design. Leading makers of enterprise solid state drives have improved their controller technology by incorporating advances that include:
—SSDs are not written to in a consistent fashion. And if a particular area is continually written to by the host, it will wear out faster. Wear leveling algorithms dynamically relocate blocks to other parts of the SSD to ensure that write activity is evenly distributed among the cells to extend device life.
—Once data is deleted from an SSD, the remaining data must be erased or moved before new data can be written to a cell, which can slow write performance. Garbage collection techniques predictively identify the best blocks on the NAND flash to erase and rewrite before the space is needed to improve write performance.
Advanced media management
—Advanced media management technology provides background media scanning, input/output error detection and correction logic, and information protection capabilities that manage reads to guard against read disturb data errors that occur when cells that surround a particular cell in a block are not rewritten and ensure the highest levels of data integrity.
Power-loss data protection—This capability ensures data retention in the event of power loss.
Figure 1: SSDs using advanced controller technologies can dramatically improve their Terabytes Written (TBW) endurance ratings
Conclusion: The Future of SSD in the Enterprise
With advances in MLC controller technology, SSD vendors have been able to improve endurance and reliability to a level that meets enterprise requirements while retaining MLC’s cost advantages. The new JEDEC standards give vendors the tools they need to demonstrate that their solutions meet enterprise demands. As a result, most industry analysts believe that MLC is the wave of the future for both clients and enterprises alike. For example, IDC predicts that while MLC accounted for only 27% of the enterprise SSD revenue in 2010, MLC revenue will rise to 52% next year and will hit 60% in 2013 .
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About the Author
Teresa Worth brings over 20 years IT Product Strategy, Marketing and Sales experience to her role as Senior Product Marketing Manager, for Enterprise Products at Seagate. Teresa is responsible for messaging and positioning the Pulsar, Savvio, and Cheetah enterprise product families and well as Seagate’s Self-Encrypting Drive security technology. Prior to joining Seagate Teresa enjoyed work at industry leaders such as Platinum Technologies, and Sterling Commerce. Teresa received her MBA from the University of Colorado – Boulder and her BBA in Management Information Systems from Texas A&M University.
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