Understanding Multilayer SSDs: SLC, MLC, TLC, QLC, and PLC

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Instead of the mechanical spinning disks of traditional hard drives, most solid-state drives use NAND flash memory, a type of non-volatile memory that does not require power to retain data once it is saved. NAND chips are also multilayered, which means they can hold more than one bit of information per cell. Drives that use these multilayered chips are classified by the number of bits between one and five each cell can hold: single-, multi-, triple-, quad-, and penta-level cell solid-state drives (SSDs).

From price to speed to reliability, multilayer SSDs come with their own advantages and shortcomings. This article compares them to help you understand the use cases where they succeed to help you decide which is right for your particular needs.

Multilayer SSDs at a Glance: Comparison Chart

Multilayer SSDs are referred to by the number of levels there are to the NAND flash chips they use, or how many bits per NAND cell. Here’s an overview of how they compare.

How Flash SSDs Work

Flash revolutionized enterprise data storage in all its forms, enabling faster boot times and application starts in PCs and mobile devices and facilitating blistering storage array performance for business analytics and other workloads. Flash SSDs quickly outshone hard disk drives (HDDs) by most performance metrics.

There are other benefits besides speed—without moving parts, flash SSDs are more durable and less susceptible to damage from abrupt movements and physical shocks than traditional HDDs. They also use less electricity. While they generally cost more per gigabyte than HDDs, the improved performance often pencils out the expense for many enterprises.

Flash memory stores data in memory cell arrays defined by Floating Gate Metal Oxide Field Effect Transistors (FGTs) which store a binary 1 or 0. Each FGT has two gates, like an electrical switch where the current flows between two points—NAND flash is named for the NOT-AND logic gate it uses. A persistent power source is not needed to retain data in flash cells. When power is cut, the FGT provides electrical charges to the memory cells, and data remains intact.

Although they have no mechanical parts that can wear out, SSDs can still fail. One measure of lifespan is the number of program/erase (P/E) cycles a drive can run before it begins to degrade and fail. Most multilayer SSDs use wear-leveling technology to combat the limited number of P/E cycles, prolonging life by evenly distributing them across all NAND cells in the drive.

Multilayer Flash: SLC, MLC, TLC, QLC, and PLC

SSDs built with multilayer NAND flash are referred to by the number of layers they contain—how many bits each cell can store.

SLC: Single-Level Cell SSDs

Single-level SSDs store one bit per cell, and are faster than any multilayered solid-state hard drive. They also have a higher endurance rate due to their ability to handle more P/E cycles than multilayered SSDs—between 50,000 and 100,000. They’re more expensive than multi-level cell SSDs, and because of their endurance and speed are often used in enterprise applications where data is constantly refreshed rather than archived.

MLC: Multi-Level Cell SSDs

Multi-level SSDs can store two bits per NAND cell, and they’re slower than SLCs because two bits take longer to read and write. MLC SSDs can produce four voltage charges—00, 01, 10, and 11—on a floating gate transistor (FGT) while using the same size voltage windows as SLC chips, but as a result, must run a time-consuming algorithm when data is stored. This algorithm makes MLC SSDs nearly four times slower for read and write operations than a single-level cell SSD. The expected lifespan is 10,000 P/E cycles.

TLC: Triple-Level Cell SSDs

Triple-level cell SSDs can hold three bits per NAND cell, and have eight different voltage states. This increases storage capacity while reducing the cost of an SSD. It also provides faster performance. But the endurance takes a hit, with an expected lifespan of just 3,000 P/E cycles for TLC SSDs. TLC NAND chips are  used in many digital consumer products.

QLC: Quad-Level Cell SSDs

Quad-level cell SSDs contain four bits per NAND cell, which produces 16 possible voltage states. Though the additional bits increase capacity and lower the cost, it comes at the expense of endurance and reliability. QLC SSDs can only sustain 1,000 P/E cycles before they become unreliable.

PLC: Penta-Level Cell SSDs

Penta-level cell SSDs can store five bits per cell, producing 32 different voltage states. Having that many bits in one space makes it more complicated for the flash controller to manage. The difference between the 32 voltage states is so tiny that error-correction software becomes overhead, lowering performance—as a result, PLC SSDs are a new technology not yet ready for widespread use. Manufacturers estimate that they will overcome the challenges within two years.

Multilayer SSD Benefits

Because of their increased capacity and lower cost, multilayer solid-state drives—particularly MLC and TLC SSDs—offer many benefits for consumers and businesses over hard disk drives. Here are a few of the most important:

• Increased durability—no moving parts to fail or generate heat
• Noiseless operation
• Lower power consumption
• Vibration and shock resistance
• Faster read times/better boot times
• Smaller form factors—more readily fit ultra-thin laptops and mobile devices

Multilayer SSD Disadvantages

Though SSDs and flash in general have been seen as marked improvements in the storage media world, there are some disadvantages to the technology when compared to HDDs.

Cost

While some users may decide that SSDs are worth the premium over mechanical hard drives, they remain more expensive. A mid-2023 market analysis found 8TB external SSDs retailing for around $539 and 8TB mechanical hard drives for around $160. The price range is closer for smaller capacity drives—1TB HDDs were retailing for about $45 and 1TB SSDs closer to $80. The price gap is narrowing over time.

Endurance and Reliability

SSDs also offer decreased endurance and reliability over time, with P/E cycles dropping as you add layers and capacity. Lifespan is measured two ways: terabytes written (TBW) and drive writes per day (DWPD). TBW indicates how many terabytes of data can be written over the SSD’s lifetime before it needs to be replaced, while DWPD is an endurance rating that tells users how often they can rewrite a specific number of gigabytes per day for a specific number of years before the SSD becomes unreliable.

Data Recovery

Data can be recovered from damaged or corrupt SSDs in some situations, but it’s not as easy or straightforward as with HDDs. Unlike HDDs, SSDs can only erase large blocks of data—if a block isn’t big enough to erase, the drive moves it before erasing it, adding write cycles and reducing lifespan. Enabling something called the TRIM command in supporting operating systems works to notify the SSD when blocks are no longer in use and can be labeled for deletion. While this extends the lifespan, it has a negative impact on recovery. Once a TRIM command has been issued, that data can no longer be recovered.

Learn more about estimating the lifespan of an SSD.

Multilayer SSD Use Cases

Enterprises looking to select and purchase multilayer SSDs have a wide range of choices. Selection criteria will depend upon intended use. Are reliability and speed key requirements? Will they be used for intense read/write operations or just long-term storage? Here are the most common enterprise use cases to help you decide.

High-Intensity Write Operations

Single-layer cell SSDs’ high endurance rate of 100,000 P/E cycles, performance, and reliability makes them well-suited to enterprise applications requiring high-intensity write operations. Though SLC SSDs cost more, the performance and reliability justifies it.

Enterprise Data Servers

The price, performance, and 10,000 P/E cycle endurance of MLC and triple-layer cell SSDs makes them the two most popular types for enterprise server use. Many businesses will use a mix, combining TLCs with MLCs to meet the DWPD requirement for write-intensive operations.

Read-Heavy Operations

Quad-level cell SSDs are best used in intense read-heavy operations involving data analytics, NoSQL databases, and artificial intelligence and machine learning (AI/ML). They’re also well-suited to streaming media and content delivery networks.

Data Archives and Long-Term Storage

While penta-level cell SSDs are not currently available on the market, manufacturers are working to address voltage challenges, errors, and durability concerns. They’re expected to be ideally configured for archiving and long-term storage applications that are not dependent on high write frequencies or speeds.

Bottom Line: What You Need to Know About Multilayer SSDs

Built upon NAND flash chips that can hold multiple bits per cell, multilayer solid-state drives outperform traditional hard disk drives on nearly every performance metric, making them desirable for enterprise uses that demand the high speeds and durability they offer. But the technology is still young, and until manufacturers continue to work out the kinks, they still have limitations—including shorter lifespans and higher costs—over not just HDDs but single-layer cell SSDs as well. Depending upon the use case, most enterprises will find a multilayer SSD to meet their needs now and, as the technology matures, in the future.

Read next: 5 Types of Enterprise Data Storage