MLC (multi-level cell) NAND Flash memories provide 2x or 3x the storage for slightly more than the cost of SLC (single-level cell) NAND Flash memories. Consequently, there’s a big economic case to be made for MLC NAND Flash devices in SSDs. The problem with that case is that there’s a big concern: MLC NAND Flash devices have a severely abbreviated write/erase cycle specification. According to a blog published in EETimes by Teresa Worth at Seagate: “Memory cells on an SSD drive have a finite write/erase life before they wear out and the drives become unreliable. 3-bit-per-cell MLC NAND flash can be erased only 100 to 500 times before it goes bad and standard 2-bit-per-cell MLC NAND can withstand only 3,000 to 5,000 Program/Erase (P/E) cycles. In contrast, maximum write/erasures for SLC NAND are 100,000, which made them more reliable.”
It would appear that the severe limitation on Program/Erase cycles would make MLC NAND Flash memories unsuitable for enterprise SSDs, which are used in applications where data reliability is everything (unlike memory sticks, where failure equals disposal).
However, writes Worth, “Newer enterprise MLC technologies, however, are incorporating improvements that enhance endurance and reliability. Advanced media management techniques, such as wear leveling algorithms that dynamically allocate blocks across the entire SSD, help ensure that write activity is spread evenly across the flash, reducing wear and tear on individual cells and extending the useful life of the SSD.”
In other words, an intelligent SSD controller with the right algorithms can make the use of MLC NAND Flash devices far more realistic even in enterprise SSDs. In addition, NAND Flash vendors are now screening MLC devices for endurance and adding on-chip error correction to make these devices far more attractive for enterprise applications. The driver here, of course, is the cost of the memories.
You can read Worth’s full blog here.