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Servers and switches 101

Why are switches used between servers and storage in a SAN environment? How do you choose the best switch? What features should one look for when selecting a switch?

Switches are used to create the "fabric" part of a SAN fabric. The reason we use switches in a SAN is to create a network so the expensive storage resources like disk arrays and tape libraries can be shared between many servers.

Storage arrays have a limited number of physical ports. If we connected every server directly to a storage port (this can be done) it would limit the total number of servers that could share the storage inside the array. Hooking up one physical storage port to a 16-port switch, allows you to connect up to 15 servers to that single physical port. This way, a single port can provide storage for many servers. This is known as the "fan-in-ratio" of the port. Figuring out how many servers should share a single port is an art in itself. Some storage vendors limit you to 16 or less. Some let you connect up to 128 servers per port.

How many servers you connect to a port depends on how many switch ports are available in your SAN. Some of the larger director class switches have hundreds of ports and the performance required by the servers connecting to it. I usually have a rule of thumb as seven servers per 1-gigabit port and 14 servers per 2-gigabit port. This is a general rule, and you should monitor the performance requirements of your servers to spread the I/O load around more physical ports if needed.

There are two types of switches: Director and modular. If you have a lot of servers to connect, choose a director since they provide more ports and include features that allow them to operate during equipment failure. Of course, this all depends on your budget, as director class switches are expensive.

Modular switches run from 8 to 32 ports (some have 64) and also have high availability features like dual power supplies and removable FRUs and they are less expensive than directors. They usually do not perform as fast as directors though. The type you choose depends on your budget and connectivity requirements.


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