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NAND and cells: SLC, QLC, TLC and MLC explained

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In today’s day and age, anything you own which serves a data storage function is likely to contain NAND Flash; its usage has seen an exponential growth over the years with petabytes (that’s one million Gigabyte) worth of products shipped over the past decade.

Nowadays, NAND can be found all around us – the smartphone you own, the server in a web hosting provider, the computer in your office, and even complex medical equipment might feature NAND.

Not all NAND is created equally, understanding the difference between NAND Flash types is an important, yet usually overlooked task by consumers. Different types of NAND have different characteristics, thereby important implications in terms of performance, endurance, reliability and cost.

What is NAND flash?

NAND Flash is a type of non-volatile storage technology. Non-volatile simply means that NAND, unlike DRAM (or system memory), does not require power to retain data. The ability to retain data after turning off the power makes NAND a great option for external on-the-go storage devices.

In contrast to hard disk drives (HDD), NAND Flash isn’t a magnetic technology, instead it makes use of electric circuits and a number of memory cells to store data. NAND holds several advantages over HDD, for instance it has no moving parts, thus in theory data won’t be affected by accidental drops or falls. NAND Flash devices tend to be smaller and lighter in comparison to HDD, but most importantly, the performance of NAND Flash devices is considerably larger than the one from HDD ones.

The major drawback from NAND comes from the fact that they tend to be expensive on a dollar per gigabyte basis, especially when compared to more traditional hard drives. The two most common ways to offset this problem are by either adding bits per cell or by moving away from 2D planar technology to 3D NAND technology and beyond.

3D NAND in a nutshell

3D NAND also known as V-NAND technology enables NAND cells to be layered up. Layering NAND contributes to overcoming planar NAND capacity limitations. As NAND cells are stacked vertically instead of horizontally higher density can be achieved without sacrificing data integrity. 

3D NAND not only offers higher memory density when compared to 2D NAND, but also is able to achieve lower power consumption, better endurance, higher read and write speeds and an overall lower cost per gigabyte.

Bits per cell

Flash memory cells are the basic building blocks of NAND Flash. Data is stored as bits in the cells, the bits represent an electrical charge contained within the cell that can be readily switched on and off by means of an electrical charge. Adding bits to the cell increases the number of states a cell can have, thereby exponentially increasing its capacity.

Additionally, the number of bits a cell contains serves as one of the primary ways to classify NAND Flash:

Single-Level Cell (SLC): They can only store one bit per cell and take up to two levels of charge. SLC NAND offers the highest performance, reliability and endurance (up to 100K P/E (program/erase) cycles). However, the memory density is the lowest among the variants and the price per GB is considerably higher than the other types. SLC is only available in 2D format and mostly used in enterprise setups.

Multi-Level Cell (MLC): MLC takes up to 2-bits per cell and four levels of charge. Available both in 2D and 3D variants, MLC offers good performance, reliability and endurance at a cheaper price than SLC. 3D NAND variants can reach P/E cycles in the range of 30K.

Triple-Level Cell (TLC): TLC stores 3-bits per cell for up to eight levels of charge. Commonly used for consumer grade products, TLC has a lower performance, reliability and endurance to the previous two. However a cheaper price and higher memory density make up for the drop in performance. The 3D variant can reach up to 3K P/E cycles.

Quadruple-Level Cell (QLC): Similarly to TLC, QLC is also commonly found in consumer grade products. QLC stores 4-bits per cell and can take up to 16 levels of charge. Among the 4 variants listed, it has the highest memory density and cheapest price. However, the lower price comes at a cost in performance, reliability and endurance (up to 1K P/E). 

Penta-Level Cell (PLC): Announced in 2019, PLC has been hailed as the logical next step in solid state storage technology. With the capacity to store 5-bits per cell and up to 32 (2^5) levels, PLC is expected to knock down HDD’s last line of defense, namely high storage capacity at affordable prices. PLC will ease the production of high capacity low cost SSDs; however the drawbacks in terms of endurance, speed and reliability found in QLC will still persist.

The cost performance paradigm

Although increasing the number of bits per cell is a surefire way to reduce costs and increase capacity, it has considerably negative effects on performance, reliability and endurance. Each additional bit makes it more time consuming to read and write from a cell, voltage needs also grow and so does power consumption. With higher voltage comes higher temperatures, which facilitate a phenomenon called electron leakage and might lead to data corruption.

Furthermore, every bit added to the cells increases the need for comprehensive error correction technologies. These solutions contribute to the maintenance of good data integrity, however they do so at the cost of higher latency and lower random performance (usually measured in IOPS).

The silver lining

Despite the drawbacks mentioned, the sizable reductions in terms of costs and an ever growing storage capacity make up for a fair trade off. Nowadays, most consumers can gain access to the blazing fast speeds of SSDs at very low price points, with in some cases prices starting from well under $100 for nearly 1TB (e.g. the Silicon Power PC60).

It is important to remember that NAND cells aren’t the only element that has an effect on flash performance, things like the interface being used, over-provisioning (dedicating a portion of the available storage to the controller), SLC caching, controller, inclusion of DRAM, among others also play an important role.

As most purchases, in the end it all boils down the consumer and its specific needs. Just as QLC wouldn’t make the cut for a 5G provider looking for storage options for their base stations, SLC solutions would be overkill for the average consumer.

Anthony Spence, Marketing Specialist at Silicon Power