Types of memories from SSD: SLC, MLC and TLC, what’s the difference?

I think we’ve all heard of the SLC, MLC and TLC memory types on the evolution of SSD. For some time there has been an investigation on how these discs can be reduced in price since they appeared on the market. 1 GB of SSD used to cost about 2 dollars making it almost impossible to buy for ordinary users. Now things have changed and 1 GB of SSD costs around 0.55 dollars, almost 4 times less.

The memory used by SSD is type NAND Flash, which, unlike the RAM is non-volatile (it does not lose the stored data when disconnected). NAND Flash memories store information in cells, which have a limited lifetime and are reflected in the use they have had, i.e. write cycles.

What does price have to do with the type of memory used by the SSD? It’s easy, the details are below:

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SLC Memory: Single Level Cell. The memory is stored in cells which can be found in only two states, 1 and 0, which is why they have the capacity of 1 bit. This allow it to have noticeably faster read/write times, since they only have to check two states per cell and therefore are less likely to have read errors and a faster response. They also offer a high durability, with about 100,000 write cycles. We have to have the capacity to 1 bit per cell and they cannot store much information which makes them very expensive.

MLC Memory: Multi Level Cell. The cell capacity is 2 bits, and can be found in four different states (00, 01, 10, and 11). The read/write speed is slower than SLC due to having to check four states. The durability is also less, between 10,000 and 3,000 cycles, depending on the manufacturing process by which it was made (10,000 for 5x nm, 5,000 for 3x nm and 3,000 for 2x nm). The price regarding the SLC is much lower, and can also store additional information. Currently it is the type of memory most used in SSDs.

TLC Memory: Triple Level Cell. Stores up to 3 bits per cell over 8 different states (000, 001, 010, 011, 100, 101, 110, and 111) and therefore can hold double the information of MLC. It is the slowest of all memories due to having variations between 8 voltages in order to check the cell status. The durability is also affected (2,500 cycles for 5x nm, 1,250 for 3x nm and 750 for 2x nm. Having so many disadvantages makes it have a truly affordable price along with a very respectable storage capacity.

  United Durability (cycles) Probability of failure Random Read Price
SLC 1, 0 100.000 Negligible 25µs high
MLC 00, 01, 10 ,11 10.000/5.000/3.000 very Low 50µs Medium
TLC

000, 001, 010, 011
100, 101, 110, 111

2.500/1.250/750 Low 100µs Medium-Low

Needless to say more, the price of SSD is not affected due to the standardization and demand (at least not 100%), but due to a reduction in memory storage capabilities that are now much less rapid and durable.

Nor should we take it as bad progress because right now the SSD is limited by SATA 3’s bandwidth and adding SLC memory increases its cost. For example, Samsung has its 840 Evo, using TLC memory, which allow its price to be cheaper and with very decent speeds, like its big brother the 840 Pro, with MLC memory.

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