How to design a core/edge SAN

Applying the PI to Brocade, Cisco and McData Switches

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SPICE makes it easy to compare several vendor configurations. For the sake of comparison, assume that a PI of seven will be achieved.
The Brocade 24000 core switch has 128 ports. Assume you chose the Brocade 3900 edge switch with 32 ports as the best choice for your SAN. This configuration would support 448 servers and 64 storage ports. There would be 16 edge switches.
C = ports on core switch = 128
E = ports on edge switch = 32
( C / 2 ) * PI = "S" or server capacity of core switch
( 128 / 2 ) * 7 = 448 servers
( C / 2 ) = number of storage ports
( 128 / 2 ) = 64 storage ports
Roundup ( S + ( S / PI ) ) / E = number of edge switches needed
Roundup ( 448 + ( 448 / 7 ) ) / 32 = 16 edge switches
The Cisco MDS 9509 core switch has a variety of configurations, making it more complex to consider. Nevertheless, in a standard core/edge configuration, only the 16-port blades will be used in each of the seven slots available. The 32-port blades couldn't be used to connect to storage devices if all storage devices are simultaneously used at full capacity. Nor are they useful for the ISLs, which are designed to match the bandwidth available to the storage ports. In this configuration, the MDS 9509 has 112 ports on seven blades with 16 ports each. To complement this core switch, a Cisco 9140 with 40 ports will support 392 servers attached to 11 edge switches. This configuration supports 56 storage ports.
C = ports on core switch = 112
E = ports on edge switch = 40
( C / 2 ) * PI = "S" or server capacity of core switch
( 112 / 2 ) * 7 = 392 servers
( C / 2 ) = number of storage ports
( 112 / 2 ) = 56 storage ports
Roundup ( S + ( S / PI ) ) / E = number of edge switches needed
Roundup ( 392 + ( 392 / 7 ) ) / 40 = 12 edge switches
The McData 6140 core switch has 140 ports. Assume you chose the McData 4500 with 24 ports as the best value to meet your availability needs. The total number of servers a single core switch would support would be 490. To achieve this, it would be complemented with 24 24-port McData 4500s. It would have 70 storage ports attached to the core switch.
C = ports on core switch = 140
E = ports on edge switch = 24
( C / 2 ) * PI = "S" or server capacity of core switch
( 140 / 2 ) * 7 = 490 servers
( C / 2 ) = number of storage ports
( 140 / 2 ) = 70 storage ports
Roundup ( S + ( S / PI ) ) / E = number of edge switches needed
Roundup ( 490 + ( 490 / 7 ) ) / 24 = 24 edge switches
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.

This was first published in November 2004

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