Micron sets its sights on quad-cell storage

NAND flash memory maker Micron announced plans for quadruple-level cell (QLC) flash memory. With 4 bits per cell, storage density will increase 33%.

Micron sets its sights on quad-cell storage
Micron

Micron is the latest NAND flash memory maker to announce plans for quadruple-level cell (QLC) flash memory, following similar announcements from Toshiba and Western Digital. It's a very technical story with a very real impact.

NAND flash memory stores data in one bit per cell, with billions of cells in the flash memory chips. For flash drives to gain capacity, there are two solutions: increase the number of chips in the drive, which has physical limitations, and increase the density per cell, which is limited by the laws of physics.

The first single-level cell, with one bit per cell, first emerged in the late 1980s when flash drives first appeared for mainframes. In the late 1990s came multi-level cell (MLC) drives capable of storing two bits per cell. Triple-level cell (TLC) didn't come out until 2013 when Samsung introduced its 840 series of SSDs. So, these advances take a long time, although they are being sped up by a massive increase in R&D dollars in recent years.

Multi-bit flash memory chips store data by managing the number of electronic charges in each individual cell. With each new cell, the number of voltage states doubles. SLC NAND tracks only two voltage states, while MLC has four voltage states, TLC has eight voltage states, and QLC has 16 voltage states.

This translates to much lower tolerance for voltage fluctuations. As density goes up, the computer housing the SSD must be rock-stable electrically because without it, you risk damaging cells. This means supporting electronics around the SSD to protect it from fluctuations.

Bottom line: This stuff is really hard to do and not cheap.

It's a catch-22 for the SSD industry. SLC drives have the greatest endurance and best voltage tolerance, but they have much lower capacity. Given the increasing use of flash, users want capacity first and foremost. I can't remember the last SLC drive I saw for sale.

Micron Enterprise SATA drives using QLC coming soon

Micron announced the plans for QCL drives at the A3 Technology Live Conference in London last, showing a slide that said Enterprise SATA drives using QLC were "coming soon." It also said NVMe drives were in the works.

That's good because there's no better way to kill SSD performance than to put it on a SATA bus. While SATA is the de facto hard disk bus used in every PC in the world, for some time now its performance has been maxed. I saw this while doing SSD reviews for Hot Hardware. Shiny new SATA SSD drives would routinely have the same performance as drives up to two years old because the SATA bus couldn't go any faster to keep up with the memory.

micron 2018 ssd portfolio Micron

To get the most out of flash memory, you need a PCI Express bus, which is where NVMe comes in. NVMe is a protocol for data transfer between the PCIe bus and flash memory chips. It offers several times the speed and bandwidth of SATA and at a few times the price. But you get what you pay for, and if you want performance, you go with NVMe. It was designed to be an enterprise SSD storage protocol. SATA was designed nearly 20 years ago as a hard disk interface.

It is believed Micron will target hyperscale data centers as its customers for the QLC drives and aim to replace high-capacity 7200 RPM HDDs. Even with a one-third increase in capacity, I remain skeptical. The other day I was at Fry's Electronics and saw a 4TB Samsung SSD drive for $1,449. The same drive capacity in standard HDD form? $149 from Seagate or WD.

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