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Vendors are starting to add solid-state storage to their arrays, but it isn't as simple as replacing enterprise drives with solid-state memory.|
EMC Corp.'s recently added support for solid-state disks (SSDs) in its Symmetrix product line is a clear sign that solid-state storage is transitioning from an exotic specialty item to a mainstream feature. Yet most enterprise array vendors aren't shipping solid-state storage products; this indicates that adding an SSD-based Tier 0 to an existing array isn't as simple as replacing traditional hard disks with solid-state drives. And while pricing for solid-state memory continues to fall, the cost of enterprise NAND flash is still much higher than that of the fastest Fibre Channel (FC) enterprise disks.
Why the increased interest in solid state by array vendors? Past solid-state offerings were mostly DRAM based, but high prices, limited capacity and the volatile nature of DRAM made these systems very expensive. Until a few years ago, the same was true for slower flash memory.
But NAND flash has one big advantage over DRAM: It's nonvolatile and holds data persistently without power, which has made it very attractive in consumer devices like USB thumb drives, cameras and mobile phones. The consumer market creates volume and, as a result, a lot of money has been flowing into NAND flash development. The result is continuous innovation in process technologies and design techniques to implement larger NAND flash devices with reduced production costs. For instance, between 2004 and 2006 most NAND flash offerings consisted of 2Gbit and 4Gbit multi-level cells (MLCs) manufactured with 90nm processes, supporting a maximum of 2bits per cell. Today, 16Gbit MLC flash manufactured with 50nm processes supporting up to 3bits and 4bits per cell are becoming the norm.
Also fueling the popularity of NAND flash memory are lower prices for the slower, less-reliable and higher capacity consumer-level MLC flash, as well as for the more powerful, lower capacity, single-level cell (SLC) NAND flash typically used in enterprise storage systems. Although enterprise-level, flash-based SSDs are still approximately 20 times more expensive than comparably sized high-end FC drives, the price erosion of NAND flash occurs on a significantly steeper ramp than it does for high-end disk drives. From a dollar-per-I/O perspective, solid-state drives are more cost-effective than disk drives, where equivalent performance can only be achieved through a very large number of spindles. "Within the next three to five years, solid-state storage will be a standard feature for most business implementations," says Mark Peters, an analyst at Enterprise Strategy Group (ESG), Milford, MA.
This was first published in July 2008