Solid State Drives use NAND technology, this makes them fast as they store bits on flash memory cell. This information is important as the number of bits that can be stored per cell is a capacity determinant. Something you want to know before you make a purchase. Generally, there are four types of flash types but let us look at two.
TLC vs QLC SSDs is the comparison of Triple-Level Cell and Quad-Level Cell flash type. The former stores three-bit per cell while the latter stores four bits per cell. What does this mean for the user? Which offers better data integrity? Which is more durable? We will explore these answers in the paragraphs that follow.
TLC vs QLC SSDs- Origin and Mode of Operation
The hardware for memory with triple-level cells was introduced in 2009 by Toshiba. Samsung then began producing NAND flash that stores information using the three bit per cell for commercial use in 2010. Samsung proceeded to produce the vertical NAND popularly known as 3D NAND with triple level cells in 2013.
For the quad level cell, Toshiba and SanDisk introduced this type of drives in 2009. In 2017, Toshiba created, the V-NAND quad level cells expanding storage capacity up to 756GB. However, in 2018, ADATA, Samsung, Intel and Micron followed suit.
TLC vs QLC SSDs- Durability and Speed
TLCs have the advantage of capacity over Single-Level Cell and Multi-Level Cell types, but this capacity comes at a price as it is slower, less reliable, and durable when compared to those two.
The QLC has a better capacity than the TLC and you guessed it, it is relatively slower, less reliable and durable. The manufacturers have not been able to eliminate the capacity for durability trade-off but as SSDs become more popular, this should be resolved.
This does not mean that the TLC or QLC will be worn out after a day or even a year, they still have a good lifespan.
TLC vs QLC SSDs- TLC
This uses the NAND flash memory type; it is non-volatile which means it can store a charge for an extended period without being connected to a power source. The data writing and processing is done via a semiconductor chip. An electronic charge erases a block (unit of data) then the disk writes new data.
TLC has three bits of data, and they are either erased or programmed. It is designed to have a floating gate transistor that is surrounded by an oxide insulation layer to store electrons. This layer also changes the threshold voltage to program a cell to zero or one.
Every SSD has an infinite P/E (program/erase) cycle, every time this cycle occurs it slows erodes the oxide insulation layer deteriorating the drive till it wears out.
TLC vs QLC SSDs- QLC
The QLC provides up to a 33% increase in storage capacity per cell when compared to the TLC. It also requires less energy to function, needs less space. For low intensity workloads, it can support up to 100 write cycles. It also provides fast read speeds for read intensive or performance-based workloads.
The QLC is useful for any data store that supports read-intensive applications, it is great for analytical applications that are used for artificial intelligence, machine learning and deep learning. For all these scenarios, data is written once before being used for accessing large data analytics.
It is also great for data archiving; it is also valuable for large data centres that stream media operations. Using QLC for said operation will result in a faster and more efficient delivery of audio and video to customers. The speed and storage capacity of the quad level cell comes with some cons.
The first is the writing durability, it is low, and it is slower when compared to the other types (SLC, MLC and TLC). Another is, as operations are carried out on the disk, the NAND cells damage gradually and just like the TLC, it has limited P/E cycles which will wear out.
This flaw makes it ideal to restrict the QLC to less write-intensive workload. Putting numbers to it, the single level cell accommodates up to 100,000 write cycles, the multi-level cell supports up to 10,000 write cycles, triple level cell supports between 300 to 1,000 and the quad level cell supports 100 write cycles.
It is an obvious trade off that resulting from trying to accommodate more bits per cell. The good thing is that when you use a 3D NAND with the level cell design it increases the number of write cycles. Using a 3D NAND, the increase in write cycles will be:
3D NAND + MLC = 35,000 write cycles
3D NAND + TLC = 3,000 write cycles
3D NAND + QLC = 1,000 write cycles
There are manufacturers that have increased the write cycles of QLC by ensuring that they wear levelling to arrange data in a way that ensures write and erase cycles are evenly distributed to memory blocks in a drive. This design also increases in lifespan of drive.
Another strategy to extend the QLC drive life span is by adding extra capacity to drive using overprovisioning. This capacity increase means that write and erase cycles to be distributed across a greater amount of flash blocks overtime. The result is an increase in the durability of the drive, an increase in space for data management and an improvement in performance.
TLC vs QLC SSDs- Choosing the right SSD
The “right” SSD is dependent on what you need the SSD for, while the QLC accommodates more bits per cell it is less durable than the TLC. So, if you are all about performance with a less intensive workload then the QLC is the right one for you.
However, if you want a longer lifespan a little more intensive workload then you should purchase the TLC. Beyond the level cell, you should also look out for connectivity (PCIe, NVMe) and it must have a DRAM for better durability.
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