Sane strategies for SAN growth


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Are smaller switches an option?
The 16- and 32-port switches that formed the basis of most storage area network (SAN) infrastructures became popular because of their relatively low incremental cost. Why not continue to expand, using this model and avoid the high initial capital expense of director-class devices? Couldn't this be viable as a "pay as you grow" option?

Using 32-port switches as building blocks, let's examine the possible growth of a small SAN (see "

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A two-tiered switch SAN design model"). A basic design assumption here is that a 10:1 ratio will be maintained between hosts and storage through inter-switch links (ISLs). The specific ratio will vary depending on the actual performance capabilities and requirements of the environment. It would be possible to add up to three more host switches (for a total of six) to this environment to support up to a total of 140 hosts (see "Basic two-tier fabric expanded"), and still maintain this ratio.

At this point, the SAN design would need to be modified. Continuing to use 32-port switches, the redesigned SAN might look like the figure called "Reconfigured two-tier fabric." This new configuration supports up to 216 host ports. The additional four switches (over the previous configuration) yield only 76 additional host connections.

Obviously, there are many other design options for expanding this SAN. However, it's clear that there's a crossover point where smaller switches are no longer economically feasible.

Beyond cost, the added complexities of maintaining and managing a mesh fabric of small switches are significant. It can be difficult to ensure consistent access to devices throughout the SAN. For instance, the need to find free ports to connect tape drives and other storage devices often results in non-optimal behavior, such as plugging into free host-side ports. The result is a jumble of servers and devices connected randomly throughout the fabric. Very quickly, any cost savings of switches over directors can dissipate due to these manageability issues. Smaller switches scale up to a point, and they are useful as edge devices, but it's critical to use some foresight to avoid these problems.

This was first published in November 2003

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