Understand SSD

MLC and SLC

There are different types of flash memory. First, there are two types of logical gates: NOR and NAND. The former is highly reliable and offers high performance levels in random access, but is very expensive and only used in small ROM modules. Since SSDs are all based on NAND-type gates, we will only talk about them in our article.

Among NAND flash memory, one can define two types of chips: MLC (Multiple Level Cell) and SLC (Single Level Cell). In MLC memories, each cell works with four levels of voltage, defining four different values, i.e. 2 bits. For SLC memories, only two levels of voltage are available, i.e. 1 bit. As a direct consequence, the density of SLC memory is 50% lower than MLC chips-based memory. So, they are more expensive to produce, but are faster in writing mode, and offer a longer lifetime with about 10 times the number of write-erase cycles of MLC memory.

In order to make SSD affordable, some manufacturers are preparing new models based on a new generation of chips entitled MLC X3, allowing the storage of 3 bits per cell, so a density 3 times higher than SLC and 1.5 times higher than standard MLC.