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Why would I use Fabric Login over Quick Loop?

Why would I use Fabric Login over Quick Loop?

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If a node is in a Quick Loop there is no need for a Fabric Login (FLOGI). Quick Loop was designed to provide connectivity into a fabric for older hub-based (FC-AL Fibre Channel arbitrated loop) solutions or an operating system that did not have FC-SW (Fibre Channel switched) Fabric support (at the time, this was HP-UX). A Quick Loop node would instead perform a LIP (loop initialization) on the FC-AL "loop," and get a loop ID to access devices within that loop. The loop IDs in the Quick Loop are placed into an address translation table (lookup table) in the switches name server so that Fabric based nodes (nodes that log in via a FLOGI via FC-SW) can have access to devices within the loop via an assigned WWN from the table. This enables communication between two different Fibre Channel protocols, FC-AL and FC-SW.

One good reason for this was for investment protection for hub-based backup solutions like Compaq's original "Enterprise Backup." The FC-AL nodes shared a loop attached tape library. Using Quick Loop enables the FC-AL hub to be connected to the switch and enabled the tape library to also be shared by F-Port (fabric based) nodes. One of the benefits of Quick Loop is that an entire switch can be set up as a hub to use the same equipment for both older and newer SANs but it also provides "point to point" access for fabric based nodes into the loop or, from one Quick Loop into another. FL-Ports (public loops) provide this same name mapping for devices on both sides of the loop. Fabric devices FC-SW have access to FC-AL devices and visa versa.

Back in the day... you could not share a tape library in a loop when disks were also being accessed on the SAME loop. This was due to sporadic LIPs occurring during tape rewind operations and would wreak havoc on disk access. This means that most nodes would need two HBAs, one for backup and one for disk access. Quick Loop solved this issue. You can now use one HBA on the fabric side of the loop to access disk in the fabric and tapes in the loop. I still recommend using two HBAs though. Use one adapter to stream backup data to tape and the other to access disks. In the case of dual HBA disk access, use a third adapter (HBA) to do backup. It will work fine with one or three, but since it's not MY budget, three will provide the best performance and still provide failover capability.

Chris

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This was first published in December 2001

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