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SSD Storage: Is SLC NAND Extinct?

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With manufacturers feeling pressure to drive down per-gigabyte SSD prices to stay competitive in a crowded market, almost all companies have begun using multiple layer cell (MLC) and triple layer cell (TLC) flash in their latest drives. Market experts say that single layer cell (SLC) NAND technology is on the way out, despite its reputation for reliability and performance.

Since SLC NAND only stores one bit per cell, it offers ambiguity as to how far it can be programmed, meaning it supports the most program/erase cycles of any NAND type. Programming MLC and TLC drives which have two and three bits per cell demands more precision with program/erase, and therefore lowers the durability rating for the drives. The tradeoff is that MLC and TLC drives offer more density at lower costs—more bits on less material means a lower cost of production and a better cost per gigabyte ratio. For a deeper dive on NAND architectures, see the SSD Buying Guide.

In the olden days of flash memory—around the turn of the decade—SLC NAND drives were commonly found in enterprise deployments, where its tolerance for program/erase cycling was desirable for high demand server computing.  Today, if you page through our enterprise SSD store, you will be hard pressed to find a single drive using SLC NAND. In fact, there is literally just one model available, the Toshiba Q300.

In the enterprise space, MLC NAND has become the dominant flash architecture, with manufacturers adding controller enhancement features to combat data loss—like self-monitoring, analysis, and reporting technology (S.M.A.R.T.), battery backup features, and overprovisioning—that extend the life beyond what the flash design provides. Pricing for MLC NAND enterprise drives have come down below $0.50 per gigabyte for 480 GB and larger drives. Market leader Samsung has used TLC NAND for its data center drives since 2014.

On the consumer side, TLC NAND and MLC NAND both remain popular choices for affordable solid state drives today thanks to shrinking die sizes. “You are going to be able to count how many atoms are in a memory node on your fingers and toes someday in the near future,” says Micron marketing director Kevin Kilbuck. Die sizes can only get so small before reaching the limits of quantum physics. This also has ramifications on endurance. With TLC NAND, “you can’t read or write as many times as with SLC or MLC.”

So why are users—enterprise and consumer—choosing MLC and TLC drives despite the endurance edge that SLC drives offer?

It has to do with the declining rate of SSD pricing combined with the rapid product cycle for drive technology. “By the time the NAND dies, the replacement cost is so low that it doesn’t even matter,” explains Jason Chen, product manager for SSD storage at Newegg. In a nutshell, the technology is moving so fast that companies and consumers alike are choosing the low cost-per-gigabyte TLC drives over the higher cost read/write endurance advantage of SLC drives.

The architecture of the future appears to be Samsung’s vertically-stacked cells (3D V-NAND) which promises an astounding 16 TB of disk space crammed into one SSD, made possible by a 48-layer, 256 Gbit NAND flash die. Samsung has not given an ETA for this mammoth drive, but the company says we should expect 4 TB versions of the 850 EVO and 850 Pro solid-state drives in early 2016. 3D V-NAND technology is proprietary to the company, meaning they are poised to be market leaders for big SSD storage at least in the short run.

Samsung’s competition is not standing idle, however. Intel and Micron have announced their own variant of 3D NAND flash, though a timeline has not yet been given for entry into the market.

A bigger question is whether companies will pay higher prices for large solid-state drives when there are comparatively inexpensive HDDs available (see: SSD vs HDD Waiting for Price Parity is Pointless). High performance value propositions do not always bode well for disk storage, something that many companies view as a commodity, and at levels above 4 TB are often used as cold storage or part of a backup solution. In this scenario, large SSD storage drives would be out of context with many companies’ data needs.

But still, you cannot deny the intrigue of solid state storage—how low can per-gigabyte SSD storage pricing go before it finally bottoms out?

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SSD Storage: Is SLC NAND Extinct?
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SSD Storage: Is SLC NAND Extinct?
Description
Market experts say that single layer cell (SLC) NAND technology is on the way out, despite its reputation for reliability and performance. Why is this?
Author
Adam Lovinus

Adam Lovinus

A tech writer and Raspberry Pi enthusiast from Orange County, California.

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