SSD's Latency Impact

In our last entry we talked about latency and what it was. We also discussed how storage system manufacturers are trying to overcome latency and performance issues of mechanical drives by using techniques like making the drives faster by using higher RPM drives, array groups with a high drive count, short-stroking those drives, wide striping those drives, and increasing the number of application servers for improved parallelism.All of these techniques cost money, are not very green, and in many cases are more expensive than simply using SSD. Not to mention that they don't typically come close to SSD performance. The result has been the existence of standalone, purpose-built SSD solutions like those from Texas Memory Systems, Solid Data Systems, and Violin Memory, or the manufacturer adding SSD in a "drive-like" manner to its current storage systems.

The speed of SSD technology, especially DRAM, changes the latency focus away from the actual storage medium, as it has now been optimized, and onto the storage system's infrastructure, which is suddenly a lot slower than the storage media. For vendors that incorporate SSD into existing drive enclosures, the performance of the shelf itself becomes a problem, the performance of the processors in the controllers becomes a problem, and an incorrectly sized cache (too big or too small) becomes a problem.

Another factor is that the software load on the controller becomes an issue. For the past several years, storage manufacturers have been piling on features to the storage controller like snapshots, replication, data deduplication, and others. All of these features take computing resources away from responding to storage I/O requests, which worsens system latency.

The result is that while the SSD technology going into the solution may be fast, simply adding SSD to your storage system may not dramatically improve performance like it should. Standalone, purpose-built SSD systems offer lower latency because these vendors have built systems from the chip up that are designed to deliver on the low latency of SSD.

In our next entry, we will examine those differences and how storage manufacturers will need to alter their delivery of SSD technology. Then we will wrap up with capacity management on SSDs. At the cost of SSD technology, the only good SSD is a FULL SSD.

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George Crump is founder of Storage Switzerland, an analyst firm focused on the virtualization and storage marketplaces. It provides strategic consulting and analysis to storage users, suppliers, and integrators. An industry veteran of more than 25 years, Crump has held engineering and sales positions at various IT industry manufacturers and integrators. Prior to Storage Switzerland, he was CTO at one of the nation's largest integrators.