Micron says it's first to QLC NAND with over 200 layers

Enhanced data density and speed upgrades, though challenges in endurance remain

Micron is now mass producing 232-layer QLC NAND, claiming it's the first memory manufacturer to break the 200-layer mark in QLC chips.

Among the different types of NAND flash, QLC is generally seen as the value option. QLC, or quad-level cell, refers to NAND that has four bits per cell, as opposed to SLC, MLC, and TLC, which have one, two, and three bits per cell respectively. The upside of adding more bits to a cell is higher data density, but comes at the cost of performance and endurance as it takes longer to fill cells with more bits and flipping more bits creates more wear.

More layers improves density so Micron naturally claims its 232-layer QLC flash is the densest in the world, beating its closest rivals by up to 28 percent, according to the memory maker. However, Micron's new QLC isn't quite dense enough to make NAND chips with 2 TB of memory, and for now the biggest chips only go up to 1 TB.

Performance has also substantially improved, at least partly thanks to a 50 percent bump in IO speed, from 1,600 megatransfers per second to 2,400, just like its 232-layer TLC NAND did two years ago. QLC is not known to be particularly fast but the 232-layer tech is rated for 7.1 GBps reads and 5.8 GBps writes. The read performance is coming decently close to the limit for an NVMe connection over PCIe 4.0, which caps out at just under 8 GBps.

As is typical with QLC, though, sustained write performance is not great. At least on Micron's 2500 SSD, which is based on the company's 232-layer QLC, write performance drops off significantly after writing roughly 10 percent of a drive's capacity, which represents the fast pseudo-SLC cache. Once that cache is exhausted, write speeds can plummet down to just a few hundred megabytes per second.

However, Micron VP Prasad Alluri did briefly mention to The Register that the company was working on a feature that could mitigate one of QLC's greatest weaknesses in a future generation.

Endurance is also a weak spot for QLC flash, and it's no different with these 232-layer chips. The Micron 2500 1 and 2 TB models are rated for 300 and 600 TB written (TBW) respectively. For reference, the 1 TB and 2 TB variants of the Crucial T705, which use Micron's TLC-based FortisFlash, respectively have an endurance of 600 and 1,200 TBW.

Micron says its aim with this latest QLC NAND was to beat competing QLC and TLC solutions in the value segment, and given the specifications, it's possible it could be very competitive. Samsung's latest 990 EVO 1 TB SSD is only rated for 5 GBps reads and 4.2 GBps writes, thanks to its 133-layer TLC, though it does have 600 TBW of endurance.

But even compared to higher-end TLC drives like Samsung's 990 PRO and Western Digital's SN850X, Micron is surprisingly close in read performance and not too far behind in writes (at peak, anyway).

With any luck, these cheap but decent NAND chips might slow down the rapidly rising prices of SSDs, but given that Micron stands to benefit from more expensive flash, we wouldn't bet that any drives using the company's latest QLC tech will rock the boat too much. ®

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