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The best way to expand a SAN

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High-availability SAN design
The University of Minnesota SAN connects three locations and ensures that storage data will remain available in case one of the three sites is unavailable.

Beyond the main data center

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SANs increasingly need to reach beyond a single data center, posing new challenges for storage architects. Disaster recovery and business-continuity requirements, as well as the need for organizations to operate in more than one geographic location, are spawning SANs that spread across multiple sites. Geographically dispersed SANs typically deploy an edge-to-core architecture, with edge SANs in smaller locations connecting back to core switches at larger sites. Obviously, segmentation and isolation of SANs is even more important in multisite SANs than it is in a single-location SAN. A problem or change in one location should never impact other locations. Depending on the size of the branch office, available bandwidth, latency tolerance of applications in use and the amount of data to be accessed, servers in remote locations are either directly connected back to storage arrays in the central location or they're attached to storage arrays in the branch office. While leveraging central storage arrays is the more cost-effective approach--eliminating the need to purchase and maintain arrays in the remote office--the cost benefit may be offset by lower performance.

Carl Follstad, manager, university data management services at the University of Minnesota in Minneapolis, went through a similar thought process when architecting the university's SAN. Follstad was faced with three locations of similar size and comparable storage needs that required him to deploy local storage arrays in each location (see "High-availability SAN design," this page). Follstad deployed a multisite SAN using three pairs of MDS 9509 directors from Cisco Systems Inc. that constitute the university's SAN backbone, spreading across the three locations. The core network forms a triangle, connecting each site with the two other sites, ensuring SAN availability of the remaining two sites if one of the three sites is unavailable. Along with three storage administrators, Follstad manages a total of 280TB of data using a combination of EMC Corp. Symmetrix DMX and Clariion CX arrays.

The University of Minnesota storage network illustrates the importance of HA in a SAN design. HA through redundancy is a prime objective of any SAN design. HA design involves reducing single points of failure at the device, SAN and site level. As the severity of an outage increases from the edge to the core, components closer to the core demand a higher level of redundancy. HA design doesn't stop at the switch and array--it goes all the way to the server. Multipath software for load balancing and automatic path failover like EMC's PowerPath and Sun Microsystems Inc.'s MPxIO enable servers to be dual-attached to redundant switches. As with performance, HA doesn't come for free and the level of redundancy needs to be balanced with cost.

This was first published in July 2006

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