NAND Flash SSD – Differences B/W SLC, MLC, TLC, And QLC

In recent years, SSDs have established themselves as the main memory to choose from for your computer, whether daily, gaming, or work. There are several features to consider when choosing an SSD. Capacity, value for money, and interface are all fundamental aspects.

However, one of the fundamental parameters concerns the memories or, better, the memory cells that comprise the most predominant part of an SSD. In this guide, we will see the differences between SLC, MLC, and TLC (as well as QLC), i.e., the different types of memories you can find in a solid-state drive. You can make your choice more informedly and buy the model that best suits your needs. 

What Is SSD NAND Flash

NAND Flash are Non-Volatile type memories, retaining data even when not powered by electricity. To give you an opposite example, take RAMs: as soon as you turn off your computer, any stored data is deleted. With NAND Flash, this is not the case. In general, NAND Flash is a memory that you can find in a whole range of different devices :

1. USB flash drives;
2. microSD ;
3. SSD drives, as we will see better in this guide.

How SSD Memories Work

As is also the case in other computer systems, within an SSD, several components work together:

1. controller, the microprocessor that manages read and write operations;
2. DRAM cache and SLC cache, which allow optimization of read and write operations;
3. NAND Flash memory is the set of memory cells in which bits of information are written.

The latter, in particular, is divided into several sections and subsections, from the chip to the word lines and actual cells. Electrical charges turn the bits on or off inside the cells. In this way, they assume the values ​​0 and 1 of the binary code. After analyzing the components that make up solid-state memories, you will understand how data writing, reading, and deletion operations take place. 

Characteristics Of NAND Flash Memory

SSD NAND Flash memories have revolutionized a sector that, until a few years ago, was the prerogative of old Hard Disks. If the latter had hard drives and mechanical parts, among the advantages of NAND Flash, it is worth considering:

1. Writing and reading are much faster operations thanks to the architecture of the memory cells and the synergy with the controller;
2. ever-increasing storage density and slowly closing the gap with mechanical hard drives;
3. Wear-leveling technologies that evenly distribute write and erase wear to extend SSD life.

In short, NAND Flash memories represent the new standard for data storage in SSDs and other devices. But they are not all the same, and the differences directly affect:

1. on SSD performance;
2. on the selling price;
3. on the duration over time.

3D And 2D NAND Flash

If you recently searched to understand which SSD to choose, you will have come across models that have 3D or 2D NAND Flash memories. But what does it mean? Both expressions refer to the way the storage cells are stacked inside the chip:

1. 2D NANDs have horizontally arranged cells that form a two-dimensional array;
2. 3D NANDs instead consist of memory cells arranged vertically in a three-dimensional matrix.

Consequently, the chip is more minor while maintaining a high memory density in the second case. In this way, it is possible to increase the capacity of an SSD without significantly increasing the occupied space or the production cost. The latter is still higher than 2D drives but is often compensated for by higher capacity.

SLC, MLC, TLC, And QLC: All The Differences

In the specific case of SSDs, NAND Flash is distinguished not only by their 2D or 3D structure but also by:

1. for the number of bits that can be stored in each Cell;
2. about the cycles of P/E, or Program/Erase, each memory cell can sustain before starting to wear out.

So let’s see the differences between SLC, MLC, and TLC memories, with a look also at QLC memories. 

SLC NAND Flash

SLC is an acronym for Single Level Cell and indicates those NAND Flash memories that can store only one bit of data per Cell. Each memory cell can only assume the value one or 0. For this reason, SSDs with NAND Flash SLC are also the fastest in data reading and writing operations. Furthermore, they are also the most resistant memories in terms of durability since they have a resistance of 100 thousand P/E cycles. 

In addition to these advantages, however, SLC memories also have a disadvantage: they are less dense than the types we will see shortly. As a result, they cost more on average and offer less capacity for the same price. The printed circuit chip will occupy a space that will not have the same storage possibilities as MLC and higher memories. These characteristics make them less ideal for consumer use but perfect for servers and industrial uses.

MLC NAND Flash

MLC is also an acronym and stands for Multi-Level Cell. Despite what the name might seem, these are NAND Flash memory cells capable of hosting two bits of information per Cell. Each Cell can assume no longer two values ​​​​(0 and 1) but four values: 00, 01, 10, and 11. The types of memories used in SSDs represent those with the best relationship between price, performance, and duration. 

Together, as we will see, in the next ones. MLC SSDs are slower than SLC SSDs and, at the same time, have a lower endurance that stands at 10,000 P/E cycles. However, the greater density of the chips allows for the creation of memory units that cost slightly less for the same capacity. For this reason, MLC NAND Flash is better suited for consumer use, especially if you value speed over storage capacity.

TLC NAND Flash

TLC NAND Flash, Triple Level Cells, has been a mini-revolution within the SSD industry. These are memory cells capable of storing three bits of data and, therefore, eight possible values. If SSDs cost much less today than they did several years ago, one reason is the use of TLC memories. They are present in almost all computer models, mini PCs, desktop computers, gaming configurations, etc. And the reason is simple: they have the best quality/price ratio combining solid performance, low cost, and good reliability. 

Of course, compared to the 100,000 P/E cycles of SLC memories and the 10,000 P/E cycles of MLC memories, the 3,000 P/E cycles of TLC memories might seem small. And, from a certain point of view, they are. TLC NAND Flash is not ideal for uses where a lot of writes and erases are made on the entire amount of memory of the SSD. On the contrary, in the consumer market, the levels of wear and tear that exist in the server and industrial sectors will never be reached. For this reason, SSDs with TLC memories are perfect for a regular computer, even for gaming or work. 

NAND Flash QLC

We close the analysis of SSD memories with NAND Flash QLC, an acronym for Quad Level Cell. If you understand how the naming system works, then you have already guessed that this typology allows the storage of 4 bits of information per Cell. There are 16 possible states, leading to a very high density, especially compared to SLC and MLC memories. At the same time, the cost per GB of a QLC SSD model is also lower. 

If you need a unit that combines higher speeds than hard drives with high capacities, then SSDs with QLC memories are the best choice. Unlike the other types, we have a resistance of 1000 P/E cycles and, therefore, a lower theoretical duration. But the same is true for TLC memories: consumer applications will not be affected much. You won’t have to worry if you use your computer for email, work, streaming, TV series, and gaming. The performance will generally be lower, but it depends on your use. 

P/E Cycles And TBW Values

It’s not easy to tell how long an SSD can last. Its useful life depends on many factors: construction, use, operating temperatures, amount of data written and erased, and so on. The P/E cycle value gives an idea of ​​what to expect from a specific type of NAND Flash memory. But that’s not all.

We must also consider wear leveling technologies and the fact that P/E cycles must be considered for each memory cell. To make evaluating the possible duration of an SSD easier, manufacturers have begun to use the value of TBW. Acronym for TeraBytes Written, the acronym indicates the amount of data that can be written to the SSD drive before potential problems. If you look at the TBW value of the various models on the market, you will notice that:

1. it is higher for SLC and MLC drives;
2. it is decreasing for the TLC units;
3. it is lower for QLC units.

However, it is always a parameter to be taken with a grain of salt. A 150TB TBW for a 250GB drive means you can write 30GB a day for nearly 15 years before encountering potential problems. 

Conclusions

As you have seen, the whole world of technology is under the NAND Flash umbrella. Even limited to SSDs, there is a difference between 3D NAND Flash and 2D NAND Flash, as well as between SLC, MLC, TLC, and QLC memories. The guide shows how each type is suited to individual performance, cost, and reliability needs. Making the correct choice depends, above all, on your needs.

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