You'll need to juggle the right number of ports on your switches and arrays to maximize performance and reduce the management complexity of your applications.
At first glance, it might seem that the number of ports is a relatively insignificant factor when choosing a storage array, switch or host bus adapter (HBA). Application requirements, operating system support, device features, performance and scalability are all factors to consider--the number of ports is almost an afterthought. But the number of ports plays an increasingly important role in the application's availability and performance, as well as the cost of the storage device and how complex it is to manage.
Think of ports as gates into arrays and switches; if these gates are congested, the fastest arrays and switches won't be able to live up to their potential. Therefore, the number of required ports into a storage system is determined by how much traffic the storage system must handle.
Proper sizing of a switch or array begins by analyzing the connected storage devices. The average and peak bandwidth requirements of each device need to be taken into consideration. Let's assume you need to size a storage switch for 25 servers that connect to three storage arrays. The device count dictates the minimum number of ports, which in our example is 28 ports. Depending on the storage bandwidth requirements of the applications running on each server, the aggregate bandwidth to the arrays
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will likely require multiple trunked connections, increasing the minimum number of ports.
Unfortunately, it's not always easy to accurately project the average and peak performance of all connected devices; the only way to get an accurate assessment is to monitor and analyze the utilization metric in production, which will reveal performance issues. If you want to add extra ports to a congested link or affix additional devices in general, it's crucial to include spare ports in the required port count. As a rule of thumb, you should plan for a port count that meets your port requirements for the next 12 months. "Typically, we see companies size their infrastructure up to a factor of twice today's needs," says James E. Opfer, research vice president at Gartner Inc., Stamford, CT. "Sizing infrastructure beyond a factor of two is expensive and, in most cases, uneconomical."
This was first published in April 2007