Enterprise Flash: MLC or SLC?
Multi-level cell (MLC) Flash is benefiting from technology advances by memory, controller and SSD vendors, while maintaining its cost advantages over single-level cell (SLC) solid state disks. This article from sister site InfoStor.com compares and contrasts single-level cell (SLC) and multi-level cell (MLC) technologies in the areas of endurance, performance and price.
Vendors are using a variety of techniques some common, all proprietary to compensate for the differences in reliability between SLC and MLC flash. Examples include wear leveling (or write leveling) to spread out the write distributions, DRAM cache and write coalescing, minimizing write amplification, over-provisioning capacity, enhanced error correction codes (ECC), compression, and other techniques for minimizing the amount of data that has to be written to the flash memory. Most of these techniques can be used with SLC and/or MLC devices, although they are becoming more prevalent in the MLC space.
"The number one issue is endurance, and theres no way to make MLC as good as SLC, but there are ways to make MLC good enough for some enterprise applications," says Jamon Bowen, director of sales engineering and customer support at Texas Memory Systems, which has been selling DRAM-based SSDs since the 1970s and introduced SLC flash-based SSDs in 2007.
SSD vendors use proprietary techniques to alleviate the endurance drawbacks of MLC flash. For example, STEC uses a NAND management technique called "CellCare" on its MLC-based SSDs. STEC also uses a proprietary technique called Secure Array of Flash Elements (SAFE), which adds parity in the SSD for recovery, reducing the error rate that a host sees.
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