Super-Sizing Data Center SANs Page 3 -

Super-Sizing Data Center SANs Page 3

Continued from Page 2

Getting Outsized SANs Under Control

The good news is that resolving SNS, ISL, convergence, and broadcast issues for large SANs has largely been facilitated by new fabric technologies such as 10 Gbps interswitch links, dynamic partitioning, and SAN routing. Using one or more 10 Gbps ISLs between directors not only simplifies the cabling scheme, but overcomes trunking and load balancing issues.

Fabric vendors have been struggling for some time over the question of how to increase performance on ISL links while ensuring in-order delivery of frames. A single sequence of frames sent over four trunked interswitch links, for example, might arrive at the destination switch in a random order. Without complex queuing algorithms, it is not possible to leverage multiple interswitch links to increase bandwidth.

Alternately, only sending sequences over specific ISLs may result in over-utilization of some ISLs and under-utilization of others. Simply replacing multiple 2 Gbps ISLs with a single 10 Gbps ISL resolves both the performance and frame delivery issues.

Dynamic Partitioning

Dynamic partitioning (pioneered by Sanera and now part of the McDATA product line) addresses the problem of swollen SNS tables by providing hardware-based segmentation of a single large director into separate SAN partitions. A 256-port director, for example, can be divided into isolated partitions that service separate departments or applications. Since each partition has a smaller SNS and operates independently of the other partitions, the convergence time for each partition is improved.

Dynamic partitioning also allows different microcode versions to be run on each partition and for individual partitions to be reset without affecting the entire switch. A software-based scheme using frame tagging (e.g., Cisco VSANs) provides separation of traffic within a switch, but does not accommodate multiple microcode versions or selective reset.

SAN Routing

SAN routing (pioneered by Nishan Systems) resolves large fabric issues by providing a layer 3 routing function for connecting SANs. Instead of building a single large fabric with the accompanying SNS, convergence, and reconfiguration issues, SAN routing maintains the autonomy of each SAN segment that is connected while simultaneously allowing designated storage conversations between SANs to occur.

SAN routing aligns with the customer requirement to share storage assets between hundreds or thousands of devices, but avoids the inherent flat network issues that large meshed SANs imply. Just as IP routing solved the problem of broadcast storms for bridged LANs, SAN routing enables customers to build very large and stable storage networks out of multiple, separate SANs.

Collectively, these new options can help storage managers achieve their business goals of sharing and managing their storage assets more efficiently and at lower cost. The requirements of Fibre Channel architecture can thus be fulfilled without forcing the customer to adapt their business objectives to the peculiar behavior of the underlying SAN technology.

Tom Clark
Director, SAN Technology, McDATA Corporation
Author: Designing Storage Area Networks Second Edition (2003) (available at, IP SANs (2002) (also available at

» See All Articles by Columnist Tom Clark

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