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Quad-level cell NAND SSDs, with their ability to store four bits of data per NAND cell, on the surface, look to be a better option than their single-level, multi-level and triple-level cell NAND counterparts. QLC NAND drives have cost and capacity advantages over SLC, MLC and TLC drives, but they come with their own set of issues. Durability and performance top the list.
Like QLC, the SLC, MLC and TLC abbreviations pertain to the number of bits of data that can be stored in the NAND cells within the SSD. SLC disks store one bit per cell, and MLC and TLC store two and three bits per cell, respectively.
Having the ability to store extra data in each cell might seem like a good thing. There's just one problem: Write operations are destructive to NAND cells. Each cell can accommodate a finite number of write operations before the cell fails. As the number of bits of data that are stored in a cell increases, the cell's durability decreases. In other words, when subjected to SDD write cycles, a QLC disk will wear out faster than an SLC, MLC or TLC disk.
According to some estimates, an SLC disk supports about 100,000 write cycles. An MLC disk only supports about 10,000 write cycles; that number goes up to 35,000 with 3D NAND. Even with the use of 3D NAND, a QLC disk only supports about 1,000 write cycles. Hence, QLC disks aren't suitable for write-intensive workloads, such as data warehousing, high-performance computing and online transaction processing.
Taking on the durability and performance challenges
QLC cells may fail after 1,000 write cycles, but that doesn't mean the entire drive will fail that fast. Storage vendors use tricks, such as wear leveling, where data is arranged so that write and erase cycles are distributed evenly among all of the blocks in the drive, to extend the life of the drives.
Overprovisioning is also used to extend the life of QLC NAND drives. Here, extra capacity is added to a drive but not made available as storage. Overprovisioning enables writes and erases to be distributed across a larger number of NAND flash blocks and pages over a given period of time, increasing durability. It also provides extra buffer space to manage program/erase cycles, which can improve performance.
Performance is another potential challenge for QLC disks; they don't perform as well as SLC, MLC or TLC disks. This is because each NAND cell holds four bits of data. Writing a bit of data to a cell that already contains data could potentially cause the cell's contents to have to be rewritten.
Some manufacturers have begun using onboard cache as a tool to overcome these performance deficiencies. But, even with onboard cache, performance can drop off considerably when large write operations are performed or when the disk begins to fill up with data. Nevertheless, even the most sluggish QLC drive should outperform an HDD.
QLC NAND drives aren't suited for all applications and workloads, but when it comes to uses such as data analytics and high-performance archival systems, they can be the right solution.