To improve and expand the capacity of flash memory, developers have generally focused increasing the number of bits that can be stored in each memory cell. But with the development of NAND flash memory and the move to stack memory cells in order to increase storage capacity, questions have risen concerning whether adding layers is automatically a good thing.
According to tech writer Brien Posey, “Early in the evolution of NAND flash memory technologies, the single-level cell (SLC) architecture was king. With SLC NAND, each memory cell can store 1 bit of information. Over time, storage vendors introduced the multilevel cell (MLC) and triple-level cell (TLC) NAND flash memory, which store 2 bits and 3 bits per cell, respectively. Quad-level cell (QLC) further increased storage capacity by allowing 4 bits of data to be stored within each cell.”
The problem, says Posey, is that as cells have become more complex, they have also become more susceptible to potential vulnerabilities.
“The first problem is related to reliability,” he notes. “As bits are added to memory cells, it becomes increasingly difficult to ascertain their value. Over the course of multiple write cycles, the cells begin to degrade, making it more difficult to accurately read bit values.”
Another problem is that while increasing the bit count increases the overall capacity of the storage media, it also makes it slower.
“But the biggest problem with adding bits to cells,” Posey says, “is that doing so causes the cells to wear out more quickly. SLC drives can endure roughly 100,000 program/erase cycles. For MLC drives, that number plummets to about 3,000 cycles per cell. TLC drives only have about 1,000 write cycles per cell, and according to some estimates, a QLC drive may only be able to endure about 100 write cycles per cell.”
Regardless of those problems, Posey says, the fact remains that while individual cells may wear out more quickly, entire drives do not. “Storage vendors employ a number of tricks to extend the longevity of their QLC drives, including wear leveling and over provisioning.”
And in the short term, he says, “expect vendors to work to improve 3D NAND and increase the number of layers within storage architectures.”