SSDs have a 'bleak' future, researchers say

The performance gap between HDDs and SSDs will shrink

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Some background

NAND flash memory chips are used to build solid-state drives (flash storage in a hard drive form factor) and PCIe-based flash cards. Over the past six years or so, the transistor size of NAND flash chips has shrunk from 72nm to 25nm, which allows more data to be stored with the same number of flash dies.

But as the size of the circuitry diminishes, so do the walls of cells that hold the electrons, which in turn represent bits of data. As cell walls thin out, electrons leak through and create data errors, which requires additional error correction code.

The first SSDs only stored 1 bit of data per NAND flash cell, which is known as SLC flash. Next came MLC flash, which offered two bits per cell. Most recently, vendors have begun producing triple-level cell (TLC) flash, which stores three bits of data per cell. The most common NAND flash remains MLC, but that's expected to change as vendors strive to produce higher capacity SSDs in order to compete with hard disk drive capacities.

Flash memory, however, wears out over time as data is marked for deletion and moved and new data is written in a process known as a program-erase cycle (P/E cycle). Special firmware in today's drive controllers more evenly spreads data throughout the drive to give the media greater endurance, but ultimately, NAND flash has a finite number of P/E cycles.

Bandwidth
Read and write MBps (bandwidth) decreases with density and capacity. (Source: UCSD Department of Computer Science and Engineering)

SLC flash has the highest reliability and resiliency, with 50,000 to 100,000 program/erase (P/E) cycles. MLC NAND can sustain 5,000 to 10,000 erase cycles. TLC NAND has the lowest endurance, with 1,000 P/E cycles to as few as 500 P/E cycles, the researchers found.

"It's a pretty dramatic decline," Grupp said. "[People] are used to working with technology that continues to just get better, but with NAND flash we're going to be facing trade-offs as it evolves."

Lucas Mearian covers storage, disaster recovery and business continuity, financial services infrastructure and health care IT for Computerworld. Follow Lucas on Twitter at @lucasmearian, or subscribe to Lucas's RSS feed . His email address is lmearian@computerworld.com.

Copyright © 2012 IDG Communications, Inc.

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