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Flash! All-solid-state arrays

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The quirks of SSD

Despite all their apparent benefits, SSDs aren’t perfect devices and they have their own peculiarities. Most significantly, flash memory has a finite write lifetime, so SSDs eventually wear out. Typically, SLC flash endures approximately 100,000 write cycles; MLC flash is an order of magnitude worse at around 10,000 cycles for individual data cells. Past those points, storing and retrieving data becomes unreliable. Manufacturers use a number of techniques to extend life, including wear leveling, error correction codes, bad block remapping and overprovisioning.

SSDs may also exhibit inconsistent response times. Data written to an SSD is spread across the entire device to ensure even wearing of individual cells. This process is known as

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wear leveling. As new data is written, blocks of cells are marked for garbage collection. As an SSD reaches a high level of utilization or write activity, the process of garbage collection can cause spikes in response times as invalid data areas are recovered. Clearly, this kind of behavior is undesirable in low-latency environments.

To emulate a hard drive or not to

Packaging flash memory in the same form factor as a traditional hard disk offers obvious benefits. It can be integrated into an array simply by replacing an existing hard drive and it’s compatible with existing protocols and form factors. Most SSD array vendors have taken this option, using readily available SSDs as their standard form factor. However, two vendors have chosen another route.

Nimbus Data Systems Inc. designed its own SSD devices, which they call enterprise flash modules (EFMs). Based on eMLC, these devices are custom-made to work within Nimbus’ S-Class or E-Class storage arrays. The EFMs have dedicated controller software and additional local nonvolatile RAM (NVRAM) that enables writes to be confirmed once completed to this faster RAM rather than waiting to have the data committed to the flash memory itself.

Violin Memory Inc. uses its own custom Violin Intelligent Memory Modules (VIMMs). These devices combine flash memory and controller logic with a three-port switch that provides redundant connections to other VIMMs in an array for high availability.

This was first published in August 2012

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