Quad-level-cell (QLC) NAND flash is reportedly debuting this year from multiple companies. If you spent 2017 frustrated at the high price of SSDs, the advent of quad-level cell NAND could be the answer to solving capacity woes. New Intel leaks show QLC 3D NAND SSDs arriving in the not-so-distant future, while Marvel showcased its own new memory controllers (which support both TLC and QLC) at CES 2018 last week.
Intel isn’t the first company to talk up the possibility of QLC NAND, but it’s interesting to see the industry moving towards the denser RAM standard so quickly. For those of you who aren’t familiar with this terminology, a quick primer. NAND is subdivided into types based on how many bits of data are stored in each physical memory cell. SLC (single-level-cell) stores one bit, MLC (multi-level-cell) stores two, TLC (triple-level cell) stores three, and QLC (quad-level-cell) stores four.
Storing more data per cell makes NAND more dense, but it also makes the memory slower — it takes more time to read and write data when so much additional information (and so many more charge states) are stored within the same cell of memory.
Back when all NAND was laid out in a 2D (aka planar) structure, TLC NAND proved difficult to scale and QLC NAND was thought functionally impossible due to extremely short cell life. The shift to 3D NAND allowed firms to use older technology in which memory cells were larger. 3D NAND at Samsung uses 40nm process technology compared with the last generation of 2D NAND, which used 20nm technology. Because NAND memory built on older process nodes has larger cells and can more easily store multiple bits of data, 3D NAND’s overall reliability and total number of program / erase cycles (P/E cycles) is correspondingly higher.
Intel is planning a new line of SSDs, the 545s, 600p, 660p, 700p, and 760p. Apart from the 545s, the new drives are all PCI Express based, with one 32-layer TLC drive and four 64-layer drives — three with TLC, one with QLC. The eyebrow-raising quotation is the performance for the QLC drive: 1800MB/s reads and 1100MB/s writes is much higher than what we would’ve expected. The BGA-based 700p is only modestly faster on claimed write performance (the highest-end 760p is much faster than both, but probably isn’t the ideal comparison).
We’re going to recommend cautious optimism as far as QLC is concerned. Tom’s Hardware reports the advent of QLC could push 512GB SSDs down to $100, which would be great, but TLC NAND had some growing pains before manufacturers ironed out the issues. QLC NAND may have a smoother ramp, but it doesn’t hurt to wait and see a bit.
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