Friday, June 27, 2014

Performance At Scale: SSDs, Silver Bullets, And Serialization

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We all love a silver bullet. Over the last few years, if I were to split the outcomes that I see with Rackspace customers who start using SSDs, the majority of the outcomes fall under two scenarios. The first scenario is a silver bullet—adding SSDs creates near-miraculous performance improvements. The second scenario (the most common) is typically a case of the bullet being fired at the wrong target—the results fall well short of expectations.

With the second scenario, the file system, data stores, and processes frequently become destabilized. These demoralizing results, however, usually occur when customers are trying to speed up the wrong thing.

A common phenomena at the heart of the disappointing SSD outcomes is serialization. Despite the fact that most servers have parallel processors (e.g. multicore, multi-socket), parallel memory systems (e.g. NUMA, multi-channel memory controllers), parallel storage systems (e.g. disk striping, NAND), and multithreaded software, transactions still must happen in a certain order. For some parts of your software and system design, processing goes step by step. Step 1. Then step 2. Then step 3. That’s serialization.

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