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Single-connection SAN

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The single-connection SAN concept is made up of the single-connection servers, eight-port edge switches with dual interswitch link connections, two spare servers, one spare switch and centralized storage in the corporation SAN.

Remove redundant connections
First, it's necessary to question whether dual SAN connections for high availability are needed on every server. Of course, many storage administrators will feel that there's no point in installing a SAN that doesn't use dual-pathing for every node, but dual-pathing effectively doubles the capital outlay by requiring duplicate host bus adapters (HBAs) and switch ports for every server.

Using a rough estimate of $1,500 per connection to the SAN, dual connections cost $3,000 per server. When you multiply the connection cost by 10 servers, it becomes a $30,000 budget item, and when you multiply it by 100 servers it amounts to $300,000. This is far from chump change.

Why even bother with the extra connection? For most servers--especially PC servers--there's more than enough bandwidth for the job in a single connection. Besides, in dual-connection configurations on PCs, the second connection will probably only be working in standby mode. In other words, there's no performance requirement for a second connection, and it doesn't get used anyway. Additionally, it doesn't matter if the connection speed is 1Gb or 10Gb--a single PC server won't come close to using all of the available bandwidth.

It may seem sacrilegious to suggest dual-connections for redundancy aren't needed, but it's important to keep the application requirements at the forefront. There are many applications that don't need instantaneous failure recovery to service clients. For instance, most Internet-based applications are designed to assume the client/server connection can be lost, forcing a reconnect.

A single-connection SAN is very reliable. Once a SAN is running, it tends to keep running. Fiber optic cables are practically indestructible except for internal tampering or an industrial accident, and it's not guaranteed that dual connections would help that much in either of those cases. HBAs and switches also tend to have excellent reliability characteristics. However, one of the components that's suspect in a single-connection SAN environment is the gigabit interface converter (GBIC). GBIC failure rates are similar to those of disk drives; in other words, you can expect them to start failing after five years of service.

Using a single-connection SAN also means there are half as many connections to manage (see "Single-connection SAN"). To the uninitiated, this might not seem like such a big deal, but it's significant to administrators who actually do the work. The fewer connections there are to check when making changes, the easier and faster it is to make them. And a single-connection SAN further reduces the number of connections per switch (by using small switches).

This was first published in March 2004

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