The Truth About Shelf Based SSDs

Posted by George Crump
September 15, 2009

Shelf based SSDs are SSDs designed to be installed into storage system manufacturers' current drive expansion shelves that they currently use for mechanical hard disk drives. It seems like a good strategy and a quick path to an SSD offering for storage manufacturers. The truth is, however, that either manufacturers or the storage manager need to be aware of the ramifications of loading up a shelf of SSD technology.

Companies like STEC and Pliant Technology offer drive slot compatible SSD drives. In fact, Pliant just announced a high performance, dual ported drive that is at the top end of performance for the category. There is nothing wrong with the technology itself, instead the challenge is going to be with the storage OEMs and how they implement the technology.

These are more than just faster hard drives, they are 15X to 30X faster, depending on the application and your read-to-write ratio. As we outline in our Visual SSD Readiness Guide, determining what applications can take advantage of SSD takes some investigation on your part, but it is not difficult. Knowing the read-to-write ratio is important in determining what SSD technology to use and what kind of performance you can expect. For example, some will provide better performance on mixed read/write workloads than others.

Storage system manufacturers have to break out of the old way of thinking; filling up a shelf with many SSD drives is not like filling it up with mechanical drives. In the mechanical world, the I/O capabilities of the shelf were never an issue. In the SSD world, a half dozen drives may max out the I/O capabilities of the shelf, add more drives, and you won't see any more performance. A full shelf SSD will still outperform a shelf of mechanical drives, but it will be nowhere near what you are paying 15X more for. I think customers are going to want to get all the performance coming to them.

Storage systems manufacturers are going to have to give customers detailed best practices guides. Do they implement half full shelves and build the array vertically up the rack? Can you then mix slower drives in the same shelf for capacity centric applications? Will the dramatic difference in performance cause issues? This is more complicated than when we were mixing SATA and fibre mechanical drives for the first time. Could it be a viable alternative to come out with lower drive slot count but high performance shelves that are only for SSD?

Lastly, what is the impact on performance of the SSD by the array software itself? SSDs may show flaws in storage software design and cause unpredictable performance as you start to take snapshots, replicate or leverage tiered migration with SSD based technology.

It is because of these questions that dedicated appliance solutions like those from Texas Memory or Violin Memory continue to do well. They are not encumbered by having to mix legacy mechanical drive technology or deal with array software overhead.

Shelf based SSDs are not bad, they make a lot of sense. However, more so than any other bandwidth upgrade, 1GBE to 10GBE for example, the additional performance is going to expose previously hidden weaknesses in the environment. The pressure is on the storage manufacturers to address the mechanical legacy of today's storage systems in order to take advantage of what SSD can deliver.

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