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Multi-Level Cell Memory Seeks Enterprise SSD Status

Price continues to be a major sticking point when it comes to the enterprise adoption of solid-state drives (SSDs), so it is not surprising that multi-level cell memory (MLC) continues to surface in industry discussions and product launches concerning SSDs. MLC costs significantly less than its single-level cell memory (SLC) counterpart.

MLC carries four bits instead of SLC's two bits. As a result, in addition to the lower price, it delivers twice the capacity of SLC and the extra capacity improves MLC memory life cycle over that of SLC. Unfortunately, MLC's multiple logic levels complicate the write process to the point where MLC delivers write endurance results in the thousands, as compared to a write endurance rating that is in the millions for SLC. MLC has a much lower write endurance rating because a complex of multiple logic levels must interact with each other, and the interaction contributes to earlier wear out than SLC. Just as significantly, MLC memory has traditionally failed sooner than SLC memory due to MLC's smaller voltage tolerance.

However, this does not prevent companies and industry analysts from projecting a role for MLC in the enterprise. "You have to believe that as the technology progresses, MLC will become capable and reliable enough for the enterprise market," says Arun Taneja, founder of industry researcher Taneja Group. He expects that MLC will assume a position in the low end of the enterprise market.

Part of this enterprise "low end" market consists of drives for notebook computers, which ST Microelectronics began shipping in 2007 and Samsung and others began shipping in 2008.

"There are still challenges in MLC performance and error rates," says Khandker Quader, senior vice president of memory technology and product development for SanDisk, which recently announced a 4-bits-per-cell (X4) MLC memory chip. SanDisk, which considers itself a vertical integration company, has pursued an approach that combines an X4 controller chip with an MLC memory chip in a multi-chip package. "The controller is a critical part of the product," says Quader. "It manages the memory for better performance and it has an innovative error handling scheme that allows us to address some of the shortcomings of MLC."

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