For SCSI you can use the OS to create mirror sets across both SCSI adapters. This is usually called "disk duplexing." If one adapter or disk fails, the OS (or file system manager software) can use the surviving path to the other mirror. I'm sure the same should apply to SSA connected disks. For Fibre connected disks in a SAN, you should use a "Path Manager" from your SAN vendor to provide redundancy to the storage. Most SAN installations use this method as a matter of fact. Why invest in a highly available SAN if an HBA outage will take the entire server down.
HDS uses software called "Dynamic Link Manager" to provide load balancing and failover in a SAN environment for up to 8 host bus adapters in a single server.
EMC uses software called "Power Path" to provide similar functionality, Compaq uses software called "Secure Path" to do the same, IBM has a similar solution to HDS and so does HP and Sun.
For ESCON, that stuff is usually handled by the mainframe attached to the DASD.
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