A: The basic rule of thumb for how many HBAs to use is two for disk and one for tape. Using 2Gb Fibre Channel across two fabrics and two 2Gb storage ports, you should be able to stream data at over 300MB/s. If you're worried about IOPS, then two connections should be fine for most data intensive applications.
The only time I have needed to use more than two adapters on a particular server is in massive digital media environments and with humongous data warehouses.
If the server itself is involved in pushing data to tape, I always specify another adapter for tape streams. If not, and you're using serverless backup through the SAN, then two adapters should be fine. Each adapter should be able to handle between 128 and 256 LUN numbers, depending on the SAN environment. If you're connecting a Windows host, you will run out of drive letters long before you run out of LUNS.
--Christopher Poelker, storage architect, Hitachi Data Systems
This was first published in January 2004