Knowing the Types of SSD: What are SLC, MLC, TLC, QLC and PLC?

SSD

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Ferdian Alfianto

Ferdian Alfianto

Ferdian Alfianto is an Internet enthusiast, Mac Lover; likes using Wordpress, experimenting with Linux (especially Debian and Ubuntu), tinkering with pfSense routers, happy experimenting with LEMP (Linux, Nginx, MariaDB, PHP) and Redis. You can contact me here.

Solid-state drive or better known as SSD, improve the performance of aging computers and turn newer computers into super fast machines. However, when we want to shop for SSDs, we will be faced with several types of SSDs: SLC, MLC, TLC, QLC and PLC. What does all that mean? 

All About Cells

Today's SSDs use NAND flash storage, the building blocks of which are memory cells. This is the base unit whose data is written in SSD. Each memory cell receives a number of bits, which are registered on the storage device as 1 or 0.

Single-Layer Cell (SLC) SSD

The most basic type of SSD is a single-layer cell (SLC) SSD. SLC accepts one bit per memory cell. Not much, but has some advantages. First, SLC is the fastest SSD type. They are also more durable and more error prone, so they are considered more reliable than other SSDs.

SLC is popular in enterprise environments where data loss is not tolerated, and durability is key. SLC tends to be more expensive, and they are not usually available to consumers. For example, I found a 128GB enterprise SLC SSD which costs the same as a 1 TB consumer-grade SSD TLC NAND type.

If you find an SLC SSD in a consumer scope, it may have a NAND type with a different SLC cache for increased performance.

SLC SSDs have a lifespan of 50,000 to 100,000 program / delete cycles.

SSD SLC
Example of an SLC SSD

SSD Multi-Layer Cell (MLC)

The term “multi-” in this MLC-type SSD is actually less precise. They only store two bits per cell, which is not very "multi-".

MLC is slightly slower than SLC because it takes more time to write two bits into the cell than just one. MLC SSDs have less robustness and reliability, as data is written to NAND flash more frequently than with SLC.

MLC has a lifespan of 1,000 to 10,000 program / delete cycles.

SSD MLC
SSD MLC

Triple-Layer Cell (TLC) SSD

As the name suggests, the TLC SSD writes three bits for each cell. At the time of writing this article, TLC is the most common type of SSD on the market.

Packs more capacity than SLC and MLC, but at the expense of speed, reliability and durability. That doesn't mean a TLC drive is bad. In fact, the TLC SSD probably is the deal best right now, especially if you are looking for a large capacity SSD at an affordable price.

Don't be bothered by the assumption that TLC SSD is a type of SSD that doesn't last; in fact these SSDs can usually be several years old.

TLC SSD
Samsung 860 EVO, TLC type SSD

Quad-Level Cell (QLC) SSD

Quad-Level Cell (QLC) SSDs can write four bits per cell. QLC NAND can pack much more data than other types, but nowadays the performance of QLC drives is under the spotlight, especially when the cache runs out during large file transfers (40 GB or higher). This may be a short-term problem, as SSD manufacturers are constantly trying to optimize QLC's performance.

Endurance is also a concern. The Crucial P1 QLC NVMe SSD is rated at only 100 TBW (Terrabytes Written) on the 500 GB model, and only 200 TBW at 1 TB. That's pretty far off when compared to TLC types, but still good enough for use on a home computer.

Intel 660p QLC SSD

Penta-Level Cell (PLC) SSD

PLC SSDs, which can write 5 bits per cell, aren't around for consumers yet, but they're on their way. Toshiba mentions the PLC drive is on end of August 2019, and Intel the following month. PLCs must be able to pack even more capacity into the SSD. However, they will have the same issues as TLC and QLC when it comes to durability and performance.

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