What are the NAND flash memory types and where do they work best?

Single-level cell

Single-level cell (SLC) NAND, as the name indicates, stores one bit in each cell. There are two states SLC NAND can be in: programmed (0) or erased (1), which depends on the level of charge that is applied to the cell. Because the number of potential states is limited to two, determining the state of a cell is a quick process.

SLC NAND is the simplest of the NAND flash memory types and has a very low chance of error. SLC NAND memory cells can take about 100,000 write operations before failure, giving them the highest endurance of the NAND types. However, it is also the most expensive option; it's more than twice the price of multi-level cell (MLC) NAND. Because of the high price, performance and reliability of SLC, it is most often used in commercial and industry applications.

Some vendors now offer drives known as pseudo-SLC. A pseudo-SLC disk is a disk that mimics SLC's characteristics by using a special controller on a disk that's equipped with a less expensive type of NAND. In doing so, the controller stores a single bit in each cell, even though the cell has been designed to accommodate multiple bits of data. The benefit of using this approach is that the disk can deliver SLC-like performance and durability, but at a lower cost. The tradeoff is that because each cell only stores a single bit, storage capacities tend to be very low.

SLC Advantages:

• Very fast read and write speeds.
• Very high levels of endurance.
• Very low error rates.
• Extremely reliable in harsh environments.

SLC Disadvantages

• High cost
• Low density / limited storage capacity.
• Primarily used only in enterprise or industrial environments.

Multi-level cell

Perhaps the most cryptically named of the bunch, multi-level cell NAND stores two bits per cell. This means that there are four possible states (00, 01, 10, 11) as opposed to SLC's two. Serving as a midpoint between single- and triple-level cells, MLC has a higher bit rate than SLC, which lowers the number of write-cycles it can sustain and increases the chance of error. Consumer-grade MLC NAND can handle about 3000 write cycles per cell.

As mentioned previously, however, MLC NAND is significantly less expensive than SLC and is the right choice for many use cases. Manufacturers of consumer-based electronic devices, such as PCs, are partial to MLC NAND thanks to its lower price tag.

However, MLC NAND is not limited to consumer-grade flash. Enterprise MLC (eMLC) is a growing and enhanced type of MLC NAND that accommodates more write-cycles than the consumer variety. EMLC NAND can handle 20,000-30,000 write-cycles, making it better suited to more demanding use cases while avoiding steep SLC costs. It is worth noting, however, that eMLC has largely been replaced by other technologies, such as 3D NAND.

MLC Advantages:

• Larger storage capacity than SLC.
• Less expensive than SLC.
• Offers a good balance between endurance and performance.
• Suitable for enterprise and prosumer devices.

MLC Disadvantages

• Slower than SLC.
• Less endurance than SLC.
• Requires more complex error correction.

Triple-level cell

Boasting three bits per memory cell, triple-level cell (TLC) NAND is another type of NAND flash memory suited to consumer-level products. Also referred to as MLC-3, 3-bit MLC and X3, TLC NAND comes with a lower price tag than both SLC and MLC NAND. There are eight possible states for TLC NAND: 000, 001, 010, 011, 100, 101, 110 and 111.

TLC has a higher storage density than SLC and MLC and a lower cost per bit. However, these come at a cost; TLC NAND has lower performance, longevity and reliability than MLC NAND. TLC NAND can handle roughly 1000 to 3000 write/erase cycles before cells wear out. TLC NAND benefits from the addition of 3D NAND, expanding its potential enterprise use cases.

TLC Advantages

• Lower cost than MLC.
• Higher capacity than MLC.
• Widely used in consumer SSDs.
• Performs very well in real-world use cases.

TLC Disadvantages

• Relatively low endurance.
• Slower write speeds than MLC devices.
• Higher error rate than MLC devices.
• Performance degrades under sustained write loads.

Quad-level cell

Quad-level cell (QLC) NAND stores four bits in each memory cell. Continuing with the previous trend, QLC NAND is cheaper than the above options, but has lower endurance for writes. QLC was developed for the additional storage capacity it provides, but is generally slower than the previously discussed NAND types, particularly when it comes to write operations.

QLC NAND is suited particularly to read-intensive applications and is used for applications supporting AI, machine learning and deep learning, where data is typically written once. It is not suited to write-intensive workloads, supporting around 800 to 1000 write cycles.

QLC Advantages

• Very low cost per GB of storage.
• High storage density.
• Good for read-heavy workloads with few write operations.

QLC Disadvantages

• Very low endurance (only about 1000 program erase cycles).
• Slow write performance.
• Requires advanced error correction.
• Not suitable for write-intensive workloads.

Penta-level Cell

As its name suggests, a penta-level cell SSD stores five bits of data in each memory cell. This extra bit per cell, as compared to QLC NAND, means that penta-level cell SSDs are denser than other types of SSDs. This increased density means that more data can be stored within the available physical space, simply because each cell is storing an extra bit (as compared with QLC).

As with the other SSD types that have been discussed, however, this increased density comes at a cost. Penta-level cell SSDs have a lower endurance than other types of SSDs, meaning that the cells are able to withstand fewer write / erase cycles before failing. Penta-level cell SSDs are also slower than other types of SSDs because of the complexities involved in distinguishing between 32 different voltage states.

Because penta-level cell SSDs wear out quickly, they are not suitable for day-to-day use. They are best suited for specific use cases involving large, relatively static data sets. For example, an enterprise environment might use penta-level SSDs for performing big data analytics or for storing AI training data.

PLC Advantages

• Highest available density.
• Lowest cost per gigabyte.
• Good for cold storage or archival use.

PLC Disadvantages

• Extremely low endurance (less than 1000 cycles).
• Very slow write speeds.
• High latency and error rates.

3D NAND

While 2D or planar NAND has one layer of memory cells, 3D NAND stacks cells vertically in multiple layers. 3D NAND SSDs are available using MLC, TLC and QLC NAND technology, but not SLC NAND. With 3D NAND architecture, an SSD has much higher density than with planar NAND, in a smaller physical space. Higher density means that 3D NAND SSDs are lower in cost per gigabyte, require less power consumption and have a higher write performance.

However, there are some disadvantages to using 3D NAND instead of 2D. The cost of manufacturing 3D NAND is higher at the outset, requiring additional steps not necessary in the production of 2D NAND SSDs. 3D NAND can be used in all the same scenarios as planar NAND, but its use will depend on the organization's requirements and budget constraints.

3D NAND Advantages

• Significantly higher capacity per chip.
• Smaller physical footprint.
• Better endurance than 2D NAND.
• Commonly used in modern SSDs.

3D NAND Disadvantages

• Higher manufacturing costs.

Brien Posey is a former 22-time Microsoft MVP and a commercial astronaut candidate. In his more than 30 years in IT, he has served as a lead network engineer for the U.S. Department of Defense and a network administrator for some of the largest insurance companies in America.