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Three storage area network (SAN) architectures have become prominent: island, collocated and core/edge. Each topology serves a particular niche, but of the three, the core/edge SAN is the most scalable and widely deployed. Designing a large core/edge SAN can be a complicated process, but the SPICE algorithm greatly reduces the complexity.
The S variable is the number of servers that will be migrated initially to the core/edge SAN. It's the milestone by which the implementation project is measured. As with any project, server and capacity requirements may change during the implementation, but setting the S goal early in the planning phase will establish a clear completion point for the project.
Because a goal of the core/edge SAN is to keep the storage port as close to full utilization as possible, the P in the SPICE equation will be equal to I (and will be referred to as the "PI"). For example, if it takes 10 servers to keep a storage port nearly full, then the same 10 servers that fill the pipe between the disk and core switch will also fill the equivalent of a pipe between the core switch and an edge switch, namely an interswitch link (ISL).
The C and E variables are largely determined by the switch's port count. The C variable will be a determining factor in the total server capacity of the core/edge topology, but the E variable isn't important. If, for example, the PI that works in your environment is seven, then seven servers will share one ISL. If you purchased 16-port edge switches, you would need three of them to support 42 servers; if you purchased 24-port switches, you would need only two. Either selection results in a capacity of 42 servers and six ISLs leading to the core switch and six storage ports.
The most challenging task in designing the core/edge SAN topology is getting the PI right. It will vary, depending on applications and platforms. Large Unix Oracle servers tend to have a lower PI than small Wintel file servers. In a SAN with a widely diverse collection of servers and applications, the overall PI might need to be broken down into the various groups that make up the SAN environment. Although it may take some effort to determine an accurate PI for your SAN, there are significant rewards for doing so.
This SPICE algorithm can be applied to a variety of vendors' hardware, so it's possible to determine the bill of materials from each vendor to compare prices. You may even use your existing SAN to determine the correct SPICE for your new core/edge SAN.
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About the author: Norman Owens is a co-founder of SNIA's Storage Networking User Group in the Minneapolis/St. Paul area.
This was first published in December 2004