Cut attenuation in Fibre Channel SANs
Rick Cook
Although SANs are designed to work over several kilometers, attenuation in the Fibre Channel connections can greatly reduce that distance and seriously degrade performance as well.
In its Redbook, "Introducing hosts to the SAN fabric," (available at www.storage.ibm.com) IBM recommends taking several steps for reducing attenuation.
Assuming the fiber is properly manufactured and properly handled in installation (e.g., no bends that are too sharp), the most potent source of attenuation problems is the connectors. All connectors introduce a certain amount of attenuation, which is allowed for in the Fibre Channel specification and installation guidelines. However other problems with the connectors can greatly increase attenuation.
To prevent problems, IBM recommends:
Have as few connections as possible on a single link.
Make sure all connectors are clean. Even tiny amounts of dirt or debris in a connector can cause serious attenuation.
Make sure all connectors are properly sealed. If moisture or other contaminants get into a connector, attenuation can go way up.
Use only one kind of fiber on a single link. Never mix different diameters of fibers.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.
