How many times have you wondered whether you could improve the performance of your storage array by adding additional cache?

Will more cache improve the performance of my storage array? This is what the vendors so often tell us, but they have no objective information to explain why it is going to help. Depending on the workload, increasing the cache may have little or no effect on performance.

There are two ways to know whether your environment will benefit from additional cache. The first is to understand every nuance of your application. Most storage managers I speak with classify this as impractical at best and impossible at worst. Even if you have an application with a very well understood workload, most storage devices are not hosting a single application. Instead, they are the hosting many different applications. It is even more complex to understand how this combined workload will be effected by adding cache.

The second way to measure cache benefit is to put the cache in and see what happens. This is the most common approach I see in the field. When performance becomes unacceptable, the options of adding additional disk and/or cache are weighed and a purchase is made. (I will save the topic of adding spindles to increase performance for a future post.) Both of these options force a purchase to be made with no guarantee it will solve the problem.

NetApp has introduced a tool to provide a 3rd option: Predictive Cache Statistics. It provides the objective data needed to rationalize a hardware purchase. Predictive Cache Statistics (PCS) is available in systems running 7.3+ and having at least 2GB of memory. When it is enabled, PCS reports what the cache hit ratio would be if the system had 2x (ec0), 4x (ec1), and 8x (ec2) the current cache footprint. (ec0, ec1, and ec2 are the names of the extended caches when the stats are presented by the NetApp system.)

Now, let’s drill down into exactly how predictive cache statistics work…

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