Multi-level SSD: SLC, MLC, TLC, QLC and PLC?

Solid-state drives improve the performance of older computers and turn newer PCs into speed machines. However, when buying an SSD, you get confused by terms like SLC, SATA III, NVMe and M.2. So what do they mean? Let's find out in this article.

1. How long can an SSD last?
2. Reasons to use SATA SSD hard drive
3. Find out how an SSD works

Everything you need to know about cell

Currently, SSDs use NAND flash memory, the building blocks of which are the memory cells. They are the basic units on which data is written to an SSD. Each memory cell accepts a certain number of bits, registered on the storage device as 1 or 0.

Single-layer Cell SSD (SLC)

The most basic type of SSD is a single-cell SSD (SLC). SLC hard drives accept a bit on a memory cell. This is not much but it has several advantages. First, SLC hard drives are the fastest SSDs. They are also more durable and less prone to failure, so are considered more reliable than other SSDs.

SLC is widely used in enterprise environment, it is important to avoid data loss and durability. SLC hard drives are usually more expensive and they are not available to ordinary users. For example, on Amazon, you can find a 128GB SLC SSD for business priced at the same price as a consumer SSD with TLC NAND.

If consumer SSD SLC is found, it could be a NAND type and SLC cache to improve performance.

Multi-Layer Cell SSD (MLC)

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The word 'multi' in SSD (MLC) is incorrect. It only stores two bits per cell, not the plurality, so much.

MCL is a bit slower than SLC because it takes a long time to write two bits into a memory cell. It is also less durable and less reliable than SLC because data is written to flash memory more often.

However, MLC is a solid SSD hard drive. Its capacity is not as big as other SSDs but you can find a 1TB MLC SSD.

SSD Triple-Layer Cell (TLC)

TLC hard drives write three data bits into a single cell. Currently, TLC is the most popular type of SSD.

It also has more capacity than SLC and MLC hard drives but is slower in speed, reliability and durability. This does not mean that the TLC drive is not good. In fact, it may be the best hard drive you can buy now, especially when you don't have a lot of budget.

LTC SSDs are less durable, but they can last for several years.

Terabytes Written (TBQW)

Usually, the durability of an SSD is expressed in TBW (Terabytes Written - the total data written in the SSD before it fails). This is the number of terabytes that can be written to the hard drive before it fails.

The 500GB model of Samsung 860 Evo (SSD popularized a few years ago) has a TBW number of 600, a 1TB model of 1200 TBW. That's all data, so such a hard drive can be used for a few years.

TBW is also an estimate of user safety, SSDs often exceed this limit. However, to ensure safety, you should back up to avoid data loss, especially with old hard drives.

Quad-Level Cell SSD (QLC)

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A Quad-Level Cell (QLC) SSD can write four bits in a memory cell. QLC NAND can pack more data than other types but currently QLC hard drives have a great influence on hard drive performance. This is especially true when the cache runs out while downloading large files (40GB or more). This may be a short-term problem because manufacturers are trying to optimize QLC.

Durability is also a concern. The NVMe QLC Crucial P1 hard drive has only 100 TBW on the 500GB model and 200 TBW on 1TB. This is a significant decrease compared to TLC but good enough for home use.

SSD Penta-Level Cell (PLC)

PLC SSD hard drive can record 5 bits per cell, no consumer model but will be available in the future. Toshiba mentioned the PLC hard drive at the end of August 2019, and Intel mentioned it a month later. PLCs can pack more capacity into SSDs. However, they have the same issues as TLC and QLC hard drives when it comes to durability and performance.

You should wait until a user reviews it and check the number of TBWs to see how long they can last and the actual number of TBW.

For example, the QLC hard drive mentioned earlier has a lower TBW number but works with about 54GB recorded per day for 5 years. No hard drive for home use can write so much data. So you can expect that hard drive to last a long time even though TBW is low.

Other SSD terms

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These are some types of NAND flash memory, but here are a few terms that may help you better understand:

1. 3D NAND : NAND manufacturers have tried to place NAND memory cells closer together on a flat surface to make the hard drive smaller and increase in capacity. This works at a time but the flash memory starts to lose its reliability when the memory cells are too close together. To solve this problem, they stack memory cells on top of each other to increase the capacity. This is called 3D NAND or sometimes NAND vertical.
2. Wear leveling technology : SSD memory starts to degrade as soon as they are in use. To help keep hard drives in good condition for longer, manufacturers have used wear leveling technology to try to write data to the memory cell as equally as possible. Instead of writing a certain block in a part of the hard drive, it will distribute data equally, so the memory cells are filled at the same relative speed.
3. Cache : Every SSD has a cache in which data is stored quickly before writing to the hard drive. This cache is important for increasing SSD performance. They usually include SLC or MLC NAND. When the cache is full, performance tends to drop significantly, this is especially true for some TLC and QLC hard drives.
4. SATA III : This is the most popular hard drive and SSD interface for PCs. In this context, the interface just means the way a hard drive connects to the motherboard. SATA III has a maximum throughput of 600 megabytes per second.
5. NVMe : This interface connects the SSD to the motherboard. NVMe moves over PCIe for fast speed. Current NVMe hard drives are about three times faster than SATA III.
6. M.2 : This is the form factor (physical size, shape and design) of the NVMe drives. They are often called gumstick because they are small and rectangular. They fit into special slots on most modern motherboards.