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Flash! All-solid-state arrays

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Designing for SSD

As noted earlier, some array vendors took the step of offering all-SSD versions of their traditional storage arrays. Although these devices are faster than their HDD counterparts, they don’t provide the same relative increase in performance that can be achieved with arrays specifically designed for solid-state drives. Traditional storage arrays were designed around the assumption that the hard drive was the slowest part of the infrastructure, so the designs implemented array-based caching and complex algorithms to place data and pre-fetch data from spinning disks. These features have no benefits when using SSD; the order of magnitude increase in SSD performance requires new architectural thinking.

One good example of this is the removal of cache from SSD array controllers. Whiptail Technologies Inc. doesn’t use any cache in its controllers, and Pure Storage Inc., another all-flash array vendor, has pushed the cache to the disk shelf. Nimbus and Violin have moved NVRAM cache to the flash module. Removing controller cache means no additional logic is required for cache coherency between controllers in high-availability configurations. This simplifies design and increases performance.

Traditional storage arrays have only recently moved to using all-SAS back-end connectivity. Fast 6 Gbps connectivity for all hard drives wasn’t essential, as traditional hard drives couldn’t fully saturate a SAS connection. However, SSDs are easily

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capable of using full SAS speeds, so all-SSD arrays need full SAS bandwidth to all back-end devices.

In addition to the wear-leveling technology implemented within SSDs to ensure even usage of all memory cells and a longer, effective life, all-SSD array vendors implement array-level wear leveling, ensuring I/O is balanced across all SSDs or flash modules to both optimize IOPS and increase the longevity of the flash.

Many vendors also make use of compression and data deduplication techniques to increase the effective capacity of their arrays. Deduplication works well with SSDs as there’s no performance penalty from the increased randomness of I/O that dedupe introduces. Pure Storage, for example, uses a range of data reduction techniques in its products, as well as variable RAID stripes and dynamic RAID rebuilds to deliver consistent I/O performance.

This was first published in August 2012

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