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Will SSD go the distance?

Posted: 23 Nov 2011 ?? ?Print Version ?Bookmark and Share

Keywords:solid state drive? enterprise? multilevel cell?

As organizations increasingly adopt data-intensive applications such as data analytics, transactional databases, geospatial applications, virtual environments, cloud computing, and more, the amount of data that must be stored is phenomenally rising. Seagate estimates that the compound annual growth rate (CAGR) for digital data stored is 50 %.

The use of these applications to manage vast amounts of data quickly and gain a competitive advantage is driving demand for high-performance solid state drive (SSD) technology in the data center. According to the Gartner Group, the enterprise SSD market grew from approximately 320,000 units and US$485 million in sales in 2009 to more than 1 million units and over US$1 billion in sales in 2010. Gartner forecasts that the enterprise SSD market will mushroom to 9.4 million units and US$4.2 billion in sales by 2015 [1].

SSDs deliver the highest-performance storage in boot speed, random IOPS and other measures critical to the enterprise. For example, while the average enterprise-class 15K-RPM hard disk drive (HDD) achieves 350 to 400 random read/write IOPS, the average enterprise-class SSD can push 50,000 IOPS for random reads and 15,000K IOPS for random writes.

Yet the remarkable performance of SSDs has overshadowed their apparent shortcomings in the area of endurance. Unlike magnetic HDDs, which can undergo an unlimited number of write/erase cycles, the number of times memory cells on the flash drive can be erased is finite. With use, flash cells wear outand when they do, the drive can become unreliable.

Client vs enterprise SSD
SSDs were first used in consumer devices, including cameras and MP3 players, and later made their way into laptop computers. While data loss is a concern for laptop and other PC users, it has far greater implications for SSDs as they move into enterprise data centers that manage an organization's most important business information. In the enterprise, unreliable storage can undermine customer satisfaction and, as an extension, revenue generation through lost business and even interrupt critical business processes.

Like HDDs, SSDs must be developed to meet the differing needs of client and enterprise workloads. Client SSDs, typically used in laptop systems serving one user no more than 8 to 10 hours per day, face far fewer rigors than data center SSDs, which operate 24x7 and must service random data patterns, highly complex reads/writes, and applications that are far more write-intensive than in any client application. What's more, enterprise SSDs must operate at a much higher temperatures than client SSDs and offer higher Uncorrectable Bit Error Rates (UBER) to ensure round-the-clock data integrity.

SSD devices from different vendors vary considerably in their ability to meet these demands. Until recently, it has been difficult to test vendor claims of SSD endurance in enterprise applications.

 SSD classes

Table: SSD classes and requirements. (Source: JESD218A, Copyright JEDEC. Reproduced with permission from JEDEC.)

In September 2010, the JEDEC Solid State Association published two sets of standards for SSD endurance and reliability. JESD218A defines endurance verification requirements for both client and enterprise SSDs. JESD219 defines workload endurance requirements for enterprise SSDs only. These standards specify requirements for each application class, describe a test methodology, and create an SSD Endurance Rating that provides a standard comparison for SSD endurance based on application class (table) [2]. The endurance rating is expressed as a Terabytes Written (TBW) value describing how much data can be written to a device over its lifetime. These standards make it easier to compare client and enterprise products.

The right SSD for enterprise
Most SSDs today use NAND-based flash memory, which retains memory even without power. The type of NAND used has historically defined its endurance. NAND-based flash memory comes in two types: single-level cell (SLC) NAND and multilevel cell (MLC) NAND. MLC has been used primarily in consumer devices while SLC is used in the enterprise for mission -critical applications due to tradeoffs between endurance and cost.

SLC One characteristic of all NAND flash is that once data has been written and then deleted from a NAND flash block, that data must be erased before any new data can be written. This program/erase process eventually breaks down the oxide layer that traps the electrons and the NAND flash wears out. With SLC, one bit of data occupies one cell of flash memory. SLC is able to withstand roughly 100,000 Program/Erase cycles or as much as 10x the number of cycles that can be handled by MLC NAND. Thus, SLC NAND offers far greater endurance. The downside is that SLC is more expensive and delivers less capacity than MLC. SLC, therefore, has been used in enterprise and niche applications where budget was not a primary concern.

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