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A look at multilevel cell SSDs

Posted: 02 Feb 2012 ?? ?Print Version ?Bookmark and Share

Keywords:solid-state drives? controller? multilevel cell?

With developments in NAND flash technology, solid-state drives (SSDs) have found their way into various enterprise and data center applications. It is important to recognize that not all SSDs are designed the same. Some devices are made for laptops, others for industrial uses, and still others for consumer desktops owned by gamers or enthusiasts, and finally, enterprise data center uses.

The challenge is to recognize that a product designed for one use or environment is not necessarily something you should use for a different environment. For example, it is not advisable to use an SSD designed for use in laptop computers in a data center environment for mission critical applications. Why? Because laptop designs are tuned and developed for the low power requirements of a battery-driven environment with low duty cycles and will be powered up and down regularly, or shut off when not in use. On the other hand, enterprise environments expect to be powered and moving data 24 hours a day, all year, for five or more years, so an SSD that can survive that demanding workload is required. The opposite is equally truean SSD designed for the enterprise may not be the best choice in a laptop.

In this article, we focus on SSD devices intended for use in demanding enterprise environments, with a focus on the attributes, features and requirements needed to support the data center for access and reliability of user data. When selecting an SSD for these applications, IT professionals have a variety of choices available, ranging from the type of NAND flash memory technology used in the drive to how intelligent the built-in SSD controller is.

The types of NAND flash memory used in SSDs include single-level cell (SLC), multilevel cell (MLC), or enhanced multilevel cell (eMLC). While SLC flash offers high endurance and fast write access times, MLC flash stores two bits per cell to offer twice the density of SLC flash, at a more cost-effective price point. eMLC flash was designed to feature the higher densities and increased endurance that MLC enjoys, however eMLC achieves this at the expense of doubling access time, making it unsuitable for true enterprise-class applications.

Table 1: A quick comparison of the NAND flash memory types.

The cost-effectiveness of MLC-based SSDs (enabling more functionality in the same footprint as SLC-based SSDs) has increased their acceptance and adoption in the data center. In its raw state however, MLC flash technology has reliability and endurance challenges, especially for write-intensive and mission-critical applications. Flash memory wears out when you write to its individual cells over time, and with the array of write-intensive and mission-critical applications in the enterprise (e-commerce, online transaction processing, social networking, cloud computing, etc.), MLC-based SSDs will wear out faster than SLC-based drives, increasing a data center's total cost of ownership (TCO), unless the right tools and technologies are implemented.

By understanding how MLC NAND flash wears and identifying the different technology tools available for optimizing SSD performance and endurance, decision makers can select an MLC-based SSD that will accelerate data access in the most cost-effective and reliable manner.

Enterprise SSD endurance
SSD endurance is defined by the measure of the usable life of the flash memory cells typically specified as the number of writes a cell can sustain. 'Writing' to a cell needs more electrical charge than 'reading' of a cell. When writing to a cell, each cell must be erased before it can be written to again. In either case, for every electrical charge that is passed through a NAND flash memory cell as part of a read or write operation, that cell will wear down.

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