To start with, FC routing permits multiple fabrics to be connected without merging them, allowing only specific devices to communicate across SAN boundaries rather than enabling unrestricted communication. While Brocade and McData Corp. coined the term Logical SAN (LSAN) and depend on multiprotocol routers to connect separate fabrics, Cisco Systems Inc. offers its Virtual SAN (VSAN) that uses FC frame tagging to segment physical fabrics into VSANs. Another way to look at LSANs and VSANs is by comparing them to zoning: While zoning provides isolation within a single fabric, LSANs and VSANs are analogous to zones that span multiple SAN fabrics.
FC routing is the least intrusive and least disruptive way of overcoming some of the challenges involved in connecting disparate fabrics. Fabrics in a logical SAN retain fabric services like name servers, zoning databases, routing tables and domain IDs, eliminating the necessity to resolve conflicts.
"We created a VSAN on a Cisco MDS 9509 Multilayer Director and attached a departmental [EMC Corp.] Clariion FC4500 array, replacing two 16-port 1Gb Brocade switches," says Follstad at the University of Minnesota. "The nice thing about VSANs is that we were able to isolate the FC4500 array without impacting our main SAN, but still had it under single management."
Performance is a key metric and the consolidated SAN needs to ensure that all apps and servers perform adequately. There are two approaches to SAN performance management. First, a SAN can be designed to meet certain performance requirements by connecting servers and arrays to the same switches and switch blades, dedicating ports to arrays and servers where needed and limiting the use of oversubscription. The second approach uses QoS to enforce performance requirements. Features like traffic classification, limiting traffic via traffic shaping, zone-based QoS, assigning more credits to certain switch ports and bandwidth reservation can ensure a required performance level. However, the more QoS features are used, the more complex SAN management will be. To avoid unnecessary complexity, Follstad decided not to use QoS, at least at this point in his consolidation project. "Instead, we designed the storage area network to have no ISL oversubscribed more than 4:1," he says. A SAN consolidation strategy obviously needs to strike a balance between the two approaches to meet SAN performance objectives.
