TLC vs. QLC SSDs: Comparing solid-state drives

In the world of data storage solutions, solid-state drives (SSDs) built from 3D NAND flash memory have been nothing short of a game-changer. While the first wave of SSDs were prohibitively expensive for storage at scale, compared to traditional hard-disk drives (HDDs), modern SSDs are faster, more durable and more reliable.

At the heart of all SSDs are the flash memory cells. Modern SSDs take advantage of 3D NAND flash technology to stack multiple layers of memory cells vertically.

Compared to older 2D NAND, which arranges memory cells in a flat matrix, vertically stacking flash cells improves data storage density, capacity and the overall cost per bit of data. Put simply, 3D NAND can store more bits of data, better.

Now capable of storing terabytes of data with improved write speeds, endurance and performance, SSDs are the go-to solution for larger storage use-cases, from high-end video editing setups to professional data centers.

But not all SDDs are created equal. Categorized based on their NAND flash memory density, SSDs that use only 2D NAND flash are known as SLC (single-level cell) drives. Subsequent innovations introduced 3D NAND, allowing for MLC (multilevel cell), TLC (triple-level cell) and QLC (quad-level cell) drives that offer better performance at increasingly lower costs per bit, depending on the situation. 

Although both QLC and TLC drives are slower than their lower cell density counterparts, manufacturers like Intel and Samsung offer optimized performance through techniques like DRAM or SLC caching.

At a basic level, solid-state drives (SSDs) are a type of storage hardware that uses flash storage to store data. SSDs typically use a type of flash called NAND flash, which stores data by using an arrangement of floating-gate transistors that function similarly to NOT AND logic gates. This combination is where the name NAND flash comes from.

Each NAND cell contains both a control logic gate and a floating logic gate, separated by a thin oxide layer. Through a process known as Fowler-Nordheim tunneling, flash memory can store binary data by using an electrical charge. The charge is sent at a voltage that traps electrons in the floating gate (representing a binary value of 1) or removes it (representing a binary value of 0).

NOR flash are flash cells that use a cell structure similar to NEITHER OR logic gates, and they are typically used for small files like boot data.

NAND flash is preferable for general storage primarily because of the following characteristics:

• Non-volatile: NAND flash can accurately and reliably retain data without a constant power source. 

• Block-based: NAND flash uses large blocks of information to read and write data. Using blocks offers better performance as opposed to a byte-by-byte approach. 

• Efficient at scale: NAND flash’s block-based structure is particularly capable at sequential computing tasks and benefits from improved efficiency and data capacity compared to NOR flash or other types of storage. 

• Durable: NAND flash is much more durable compared to physical storage devices like floppy diskettes or HDDs, which require moving parts that can break.

The first SSDs were designed for compatibility with the SATA (serial ATA) memory storage interface, a legacy interface inherited from older HDD technology. However, most modern SDDs use the newer non-volatile memory express (NVMe) protocol to take advantage of the high-speed peripheral component interconnect (PCI) or PCI express (PCIe) interface and achieve even better performance.

About 20 times faster than SATA SSDs, NVMe SSDs are able to achieve data transfer rates from 3,500 MB up to 14,000 MB per second. In addition to increased throughput, NVMe drives can perform thousands of simultaneous command queues and also consistently outperform SATA drives in latency testing.

For these reasons, NVMe SSDs have quickly become an industry standard for consumer electronics and high-performance professional applications.

Ultimately, evaluating the value of TLC drives versus QLC drives can come down to a comparison between performance and durability or storage capacity and cost effectiveness.

Generally, TLC SSDs are preferable in situations where consistent performance, reliability and rugged durability are deciding factors. TLC SSDs can be trusted to reliably write and retrieve data again and again. They are frequently chosen for professional workloads and content creation. 

Conversely, QLC SSDs are favored for high-capacity, low-cost storage and read-heavy data warehousing. These types of SSDs are trusted to store long-term data that might be accessed often. However, in situations demanding increased write cycles, where data might need to be written and rewritten more than simply read, TLC SSDs might be preferable for better write endurance over longer lifespans. 

In terms of endurance, SSDs are given a TBW (terabytes written) rating to indicate the amount of data that can be written to a drive before wear and tear. The rating shows when deterioration of the physical hardware might start to impact performance and operation negatively. Incurring more wear from larger amounts of stored data on each cell, QLC drives usually have lower TWB ratings compared to TLC drives.

It is worth noting that TLC drives currently have an edge over QLC in terms of durability. To counter any disadvantages related to the lower endurance issues common to QLC drives, improvements in error correction algorithms are closing the gap between these types of NAND flash drives.

At a glance, the differences between QLC drives and TLC drives can be summed up based on architecture, lifespan, performance and use-cases.

• Storage architecture: Uses TLC NAND flash memory that holds 3 bits of data per memory cell.

• Performance: Although more expensive than QLC, TLC offers a good balance between capacity, price and write and read speeds. Generally, they are more durable and versatile and they are optimized for frequent read and write operations and more write-intensive tasks.

• Use cases: Use cases include demanding tasks like gaming, 3D rendering and other types of content creation. 

• Lifespan: TLC drives typically have a lifespan of around 1,000 to 3,000 write cycles.

• Storage architecture: Uses QLC NAND flash memory that holds 4 bits of data per memory cell.

• Performance: Offers larger storage capacity at lower prices. Increased capacity comes with some tradeoffs in performances and lifespan longevity. QLC drives typically offer better overall read performance than write performance. They are well suited for archival storage of data that might need to be referenced, but not frequently edited or erased and rewritten.

• Use cases: Ideal for lighter tasks that might require more read operations than write operations like storing large files or backups long term. Ideal for tasks that can benefit from cheaper, higher capacity storage that can tolerate decreased speed, such as media storage, light daily storage or computer backups. 

• Lifespan: QLC drives usually have a lower lifespan of around 1,000 write cycles compared to TLC drives, as increased data capacity wears out memory cells faster.