Unsnarl port traffic


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Midsized arrays
Choosing the right array is a complex process that's largely determined by application and scalability needs. One of the key decisions you'll face is whether you need the performance, features, modularity and scalability of a high-end array or if a midsized array suffices. Midsized arrays are typically dual-controller arrays with a host port count of four to eight ports, as well as two to eight back-end ports.

While a single, back-end port per controller is sufficient for entry-level arrays to redundantly attach disk enclosures, two host ports per controller are imperative to enable cluster-type configurations with two servers or switches attached to each controller.

In addition to the number of ports and port speed, arrays differ in how the back-end ports are connected to the disk enclosure. Vendors have been transitioning from an arbitrated loop design with shared bandwidth to a switched or point-to-point-type architecture to connect disks to controllers. Switched connectivity not only provides higher performance than a loop, "it also simplifies dealing with individual disk drive failures," says Craig Butler, manager of disk, SAN and NAS product marketing at IBM Corp. "That's why all our enclosures have a switched back end, all the way from the storage controller to the enclosure and drives."

For backward-compatibility reasons, some arrays ship with a hybrid loop/ switched architecture. For instance, EMC

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Corp.'s recently released Clariion CX3 UltraScale series arrays attach the disk enclosure via an arbitrated loop, but the disks within the enclosure are connected via a switched connection that enables the UltraPoint enclosures to work with pre-CX3 UltraScale series arrays. Similarly, the Hewlett-Packard (HP) Co. StorageWorks Enterprise Virtual Array (EVA) family deploys a hybrid interconnect to the spindles.

"We use a point-to-point connection between the controllers and the disk enclosure; but within the enclosure itself, the drives are connected via an arbitrated loop," reports Kyle Fitze, director SAN marketing, HP's StorageWorks Division.

While midsized arrays from EMC, HP and IBM have a relatively fixed number of host ports--from four to eight, depending on the model--and a relatively fixed back-end port count (two to eight), these arrays don't allow you to mix host-side and back-end ports. Network Appliance Inc. arrays, on the other hand, let you configure each available array port as target (host port) or initiator (back-end port), thereby giving storage managers the option to designate any number of ports as host or back-end ports.

This was first published in April 2007

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