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Solid State: New frontier for storage

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Architectural options
Before diving into solid-state storage design options, it's important to define Tier 0 so you can categorize a vendor's design as a Tier 0 implementation. A key criterion of Tier 0 is its ability to plug into existing tiers and participate in data movements between Tier 0 and other tiers. This clearly excludes cache--both DRAM and NAND flash-based caches--from being considered as Tier 0 storage. Not all array vendors add solid-state storage as a new tier; some, like NetApp, simply add it as a solid-state cache. In addition, Tier 0 is the fastest of all tiers, which makes it questionable why the term Tier 0 had to be coined in the first place. Tiering is very use-case specific and one company's Tier 3 could be another company's Tier 1.

Solid-state storage is finding its way into storage systems in a variety of ways:

Solid-state disks that replace hard disks are the easiest way of adding solid-state storage to an existing array. "The biggest challenge in adding solid-state storage to the Symmetrix array family was ensuring seamless integration between the solid-state tier and other tiers, and making sure that all features continue to work flawlessly," says EMC's Wambach.

First-generation solid-state implementations like the EMC offering add SSD without changes to the array architecture. Because arrays were designed for hard disk performance, array

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controllers are becoming the bottlenecks. Even high-end arrays like the Symmetrix DMX-4 could be pushed against its performance limits if too many solid-state drives were added. Therefore, array sizing and putting the right number of solid-state drives into the array is instrumental to ensure predictable overall array performance. "As solid-state storage becomes more prevalent, array vendors will redesign their arrays to be able to better cope with the high performance of solid-state drives," explains IBM's Barrera.

Solid-state storage as cache is the architecture chosen by NetApp in its first generation solid-state offering. "Solid-state storage is challenging and unproven in enterprise storage systems," says Chris Bennett, NetApp's VP of core systems. "Therefore, we decided to go with a more conservative approach and use solid-state storage as cache only."

More specifically, NetApp will use solid-state memory to cache meta data. By storing a copy of the meta data on a NAND flash PCI Express card in the storage controller, meta data can be accessed at memory speed and with data only needing to be fetched from disk drives. The result is a significant performance boost. "By accessing meta data from solid-state memory, we're seeing a 40% performance gain for applications like Exchange," reports Bennett.

Like NetApp, Gear6 sees the near- to mid-term role of solid-state storage as cache rather than a disk drive replacement. "Using memory as a persistent storage device is more trouble than it's worth because of very different management requirements," says Gary Orenstein, Gear6's VP of marketing. The firm's Cachefx appliance provides a pool of DRAM or NAND flash that sits between clients and NAS devices to speed file access. Cachefx is accessed like a NAS device, except it currently supports only the NFS file-system protocol.

This was first published in July 2008

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