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Unsnarl port traffic

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Port strategy comparison

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High availability
One of the strategies in designing redundant storage systems is to connect each storage node via dual or multiple paths with the next node. This is accomplished by connecting servers and storage arrays to separate fabrics. For the highest level of availability, the redundant ports on each storage node typically originate from physically separate components, eliminating downtime due to component failure. For storage arrays, this means having primary and secondary ports on separate controllers; in the case of switches and directors, this is achieved by putting ports in the same multipath group on physically or virtually separate fabrics; on the host-side, ports on separate HBAs result in higher availability than having ports in the same multipath group on a single multiport HBA (see "Ports and Fibre Channel HBAs").

A redundant storage design doubles the number of required ports and significantly increases storage costs. A mandate to lower storage costs without compromising availability has caused some storage managers to reduce the number of ports by eliminating storage switches, moving from a networked storage architecture to a more directly connected design. "With a dual-controller Clariion AX150 priced at about $10,000 and FC switches priced at about $5,000, being able to build a highly available environment without switches can literally cut the cost in half," says Jay Krone, director of Clariion platforms at EMC. However, a directly connected architecture isn't the norm for large storage environments; the more servers you need to connect to an array, the more you can make a case for SAN switches.

Ports aren't necessarily the primary concern when buying an array or switch, but they play an instrumental role when building a balanced storage system (see "Port strategy comparison," this page). As a general rule, the number of required ports is determined by the amount of data per second a storage system needs to handle; and your system will only perform well if it's in balance with the system's internal capacity.

This was first published in April 2007

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