Multi-level cell flash is a type of flash memory that stores more than 1 bit per cell. MLC flash is less expensive than single-level flash (SLC), which makes it a popular choice for consumer-grade solid state storage (SSS).
MLC flash has a higher bit error rate than SLC flash because there are more opportunities for misinterpreting the cell’s state. Unlike single-level cell flash, which only stores 1 bit per cell and is always in one of two states, programmed (0) or erased (1), MLC flash cells have more than two states because each bit in the cell is either programmed or erased. MLC-2, for example, has four states. MLC-3 has eight states and MLC-4 has sixteen; as with SLC flash, each state is determined by the level of electrical charge that’s applied to the cell.
In general, the more bits the cell has, the fewer write cycles it will have. For example, a 2-bit MLC cell is good for about 3,000 to 10,000 write operations before it begins to fail, while a 3-bit MLC cell would only have 300 to 3,000 write cycles. Once a cell is written to its limit, the cell starts to forget what is stored and the data can become corrupt. Vendors are working to overcome MLC’s limited number of writes by building smarter flash controllers.