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Solid-state storage update

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Challenges of NAND flash and SSDs

Unlike DRAM, NAND flash is non-volatile, capable of retaining data as hard disks do, but without depending on vulnerable mechanical parts and requiring significantly less power. But these benefits are offset by shortcomings the storage industry has been attempting to address for several years:

  • Durability issues with NAND flash
  • Low write performance of NAND flash
  • Inadequate software to efficiently support solid-state drives
  • Architectural shortcomings of storage systems that have been designed for mechanical disks

Durability issues of NAND flash

The most severe issue with NAND flash is the wear-out of cells, which limits a cell's life span to a very finite number of writes. While consumer-grade multi-level cell (MLC) flash permits approximately 10,000 writes per cell, enterprise-grade single-level cell (SLC) flash supports about 100,000 writes per cell before becoming unusable. The wear-out problem worsens as density increases. The 10,000 number of supported writes of 2-bit per cell MLC flash used in consumer-grade products looks generous

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vs. the newer 3-bit per cell offerings with their 1,000 to 5,000 supported write cycles and the few hundred supported writes of 4-bit per cell flash. The data storage industry has been contending with this simple rule of NAND flash: as density increases, both cost and durability decrease.

With SLC NAND flash now capable of meeting enterprise storage requirements and accepted in the enterprise space, storage vendors are trying to further decrease its cost by bringing MLC flash into the enterprise realm. Specifically, they're looking to use 2-bit per cell multi-level cell flash to compete with single-level cell flash, and 3-bit and 4-bit per cell flash for read-intense applications with scant write requirements, such as data archival.

"It's not a question if MLC flash can be used in enterprise storage systems, but a question of what it takes to make it happen at an acceptable cost," said Mark Peters, an analyst at Enterprise Strategy Group (ESG) in Milford, Mass. There are already a few instances where MLC flash is used in the enterprise space today. The most prominent example is the Hewlett-Packard (HP) Co. StorageWorks IO Accelerator for HP BladeSystem c-Class, a direct-attached, solid-state storage array mezzanine card; the HP product is based on Fusion-io's ioDrive, and uses both SLC and MLC flash depending on capacity.

Enterprise-grade solid-state drive vendors have employed a variety of techniques that enable their products to match and even exceed the life span and durability of mechanical disk drives. With SSD drives warranted for three to five years -- depending on the SSD vendor -- and mean time between failures (MTBF) north of 1 million, enterprise-grade SSD drives are at least as durable as high-end disk drives.

"By now, we consider SSD drives as reliable as high-end FC drives," noted Claus Mikkelsen, chief technology officer, storage architectures at Hitachi Data Systems. To attain this degree of durability, sophisticated wear-level algorithms that reduce the number of writes and distribute writes evenly among flash cells have been devised and implemented in solid-state drive controllers. The use of spare capacity, which typically ranges from 20% to 100% of usable capacity, extends the life span of SSDs by reducing the number of times cells are written to during a given time period and providing the extra capacity to replace defunct cells. Compression and data deduplication algorithms are used to maximize efficiency and reduce the number of writes per cell. And similar to high-end mechanical disks, enhanced error-correction algorithms are used to find, fix and isolate bad blocks. "Error-correction codes [ECCs] used to occupy four or five bits per 512 byte block; now six to eight bits are common and we're seeing it move to 12 bits," Gartner's Unsworth explained.

This was first published in September 2009

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