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What Is MLC?

A multi-level cell (MLC) solid state drive (SSD), also called a double-level cell (DLC), is a memory cell that can store more than a single bit of information, as opposed to a single-level cell (SLC), which can store only one bit per memory cell.

MLC refers to cells that can store two bits of information, as opposed to triple-level cells (TLC) and quad-level cells (QLC), which can respectively store three and four bits of information per cell.

What are the advantages of MLC SSD?

The primary advantage of MLC flash memory is its lower cost per unit of storage due to the higher data density. Because MLC SSD flash memory is generally less expensive than SLC, it is the preferred solid-state storage memory of manufacturers of consumer-based electronic devices.

What are the disadvantages of MLC SSD?

Typically, as the 'level' count increases, performance (i.e., speed and reliability) decrease. This is because it requires more processing power to read more information and also more accuracy because the more bits of information there are to read, the smaller the margin of error. As such, MLC typically has a larger bit error rate than SLC.

Other drawbacks of MLC SSDs, compared to SLC SSDs, are lower write speeds, lower number of program/erase cycles, and higher power consumption. Read speeds can also be lower for MLC NAND than SLC due to having to read the same data at a second threshold voltage to help resolve errors.

How does MLC SSD work?

Every memory cell has a certain number of possible “states” that correspond to the presence, or absence, of electrons on the microchip’s charge trap. Every state is represented by ones and zeros according to the voltage level on the trap. SLC has two possible voltage states (0 and 1) to store one bit per cell.

MLC SSDs, since they store two bits per cell, have four possible 0/1 combinations (00, 01, 10, and 11) from four possible voltage states. TLC, which stores three bits of information in each cell, is made possible by charge traps with eight different potential electron levels resulting in eight different possible corresponding threshold voltages. QLC can store four bits of data using 16 states, and so on.

How Reliable is MLC SSD?

MLC SSDs are generally considered to be less reliable than SLC SSDs because data is written to the NAND flash more often than with an SLC, leaving more chances for errors. However, this may not necessarily be true in practice. One six-year study from the University of Toronto and Google found that SLC and MLC SSDs failed at roughly the same rate, despite MLCs having a significantly lower write durability.

The study also found that:

  • Age, and not the number of writers, was the biggest reason for failure of the SSD.
  • Uncorrectable bit error rate wasn’t useful at all, but raw bit error rate was. 
  • 30 to 80 percent of SSDs develop at least one bad block and 2-7 percent develop at least one bad chip in the first four years of deployment, meaning that while SSDs are unlikely to lose all their data at once, the way HDDs do, they still tend to lose snippets of data.

Which SSDs are MLC?

It’s a common question customers will ask when sourcing their SSDs. MLCs are SSDs that store 2 bits per cell. 

Some of the most popular ones include:

The Bottom Line: MLC SSDs Balance Capacity and Reliability

In summary, storing more bits per cell has traditionally translated to a tradeoff between reliability and capacity. Storing two bits of data per cell, MLC SSDs deliver capacity and reliability ratings between SLC cells and TLC cells on the market. As technology advances, this reliability gap between MLC and SLC cells has been significantly reduced where the age of the cell is a bigger contributing factor. For this reason it’s important to understand the full performance, reliability, and capacity ratings of a data storage solution as a whole before making a decision when purchasing storage.

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