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Scaling storage might seem as easy as tossing a few more disks into the array, but adding just capacity can affect your overall performance.
"Scalability" is often defined as the ability of a storage system to support more or higher capacity hard drives. But that's not the whole story. For a storage array to be considered truly "scalable," there are other factors that are just as important -- or maybe even more important -- than disk capacity. Scaling an array's disk capacity may be as simple as buying a few drives, but scaling throughput or performance (such as changing the fan-out ratio or an application I/O profile) can be a challenging task. And it's a task that may be compounded if too little thought was put into the initial design of the storage system in terms of how it was implemented or its hardware configuration. There are many perils associated with growing a storage environment, as additional I/O may cause an imbalance that could impact the system's overall performance.
When additional capacity is being considered, you should first ensure that the additional disk won't push the system beyond its scalability margins. For example, with some modular arrays that use back-end arbitrated loops, loop contention creeps in after a certain number of disks are added and performance is affected.
Much of the confusion over scalability vs. capacity can be attributed to storage vendors not providing adequate information when pitching the virtues of their systems. More often than not, the vendor sees the sale of an array as a point solution to address a customer's immediate storage needs. So it's important that users articulate not only their present requirements, but their anticipated future growth and scalability needs. Users should also ask to see the vendor's roadmap and determine if the vendor's plan will preserve their investment over the next few years.
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