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Status report: Solid-state storage

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Solid-state storage has carved out a niche in the storage ecosystem, establishing itself as a viable alternative for high-performance applications.

Solid-state disk is, of course, nothing of the sort. Whereas a disk is a round, flat object, solid-state storage is really just memory chips. That may seem like a silly semantic distinction, but it’s actually important to bear that in mind when architecting a data access solution. Solid-state drives (SSDs), also referred to as flash memory and flash cache, have more in common with memory -- specifically cache memory -- than with spinning hard disk drives (HDDs). Although SSDs are commonly deployed “behind the storage-area network (SAN)” and provisioned as part of the total storage pool, they behave like large repositories of cache. That’s important to consider when designing solid-state storage into a storage solution.

SSD chip technologies

Three solid-state storage technologies dominate the market today: single-level cell (SLC), multi-level cell (MLC) and enterprise multi-level cell (eMLC). This may seem like an “inside baseball” discussion, but you’ll need to understand the different SSD technologies (just as you do with HDD technologies) to make the appropriate deployment choice.

MLC is currently the most prevalent consumer-grade solid-state storage, whereas most enterprise-class products are built around SLC. MLC offers a significantly lower price point on a per-GB basis but

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also has a significantly lower useful life. Individual SLC memory cells can sustain approximately 100,000 write operations before failure, but MLC cells are only good for about 3,000 to 10,000 write operations before they fail. Cell failure can be one cause of SSD performance degradation and may be the reason a solid-state device gradually becomes unacceptable over time. Obviously, MLC devices will retain their capacity and performance only about one-tenth as long as SLC for a given write workload. It’s therefore important to ask the vendor to describe the “use profile” for their product and to factor it into the cost-per-unit equation. A product that looks like an irresistible deal at half the price of other systems is no bargain if its useful life is just a small fraction of the higher priced product.

Enterprise MLC is a newer technology gaining traction in the industry. With an estimated life of 20,000 to 30,000 write cycles, eMLC reaches a middle ground both in price and life span between SLC and MLC. Nimbus Data Systems Inc. has committed to eMLC technology, using it in all its data storage products while other vendors are still using SLC. To avoid write-related lifespan and degradation issues, Nimbus’s controller software has “wear-leveling” capabilities and aligns write blocks with flash cells. Nimbus also offers a five-year warranty for those concerned with product longevity.

Server-based solid-state storage

Another emerging technology trend is toward host-based solid-state storage delivered as PCI Express (PCIe) cards for insertion directly into the host. Fusion-io Inc., LSI Corp., Texas Memory Systems Inc. and Viking Modular Solutions all offer PCIe solid-state products. Although provisioned like storage, host-based solutions behave very much like cache. Being “in front” of the SAN has the advantage of avoiding network latency for read operations, yet data can be pre-positioned using automated storage tiering (AST) technologies, depending upon the array vendor’s capabilities. On the flipside, it’s subject to host failure so storage managers should ensure the PCIe solid-state storage is properly data protected through RAID, mirroring or clustering.

EMC Corp. is joining the fray and has announced its “Project Lightning,” which is its first PCIe host-based storage product (scheduled for availability later in 2011). This product will be fully accessible using EMC’s Fully Automated Storage Tiering (FAST) software so that it works seamlessly with EMC’s arrays across the SAN. The initial product will be based on SLC technology to maximize the longevity, performance and reliability of the device.

SOLID-STATE DEFINED

Enlarge SOLID-STATE DEFINED diagram.

This was first published in October 2011

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