Ah, the beauty of standards! What makes auto discovery work well in a SAN is that the SNIA and the Fibre Channel industry alliance have developed driver standards that can be implemented by HBA and device vendors so individual software vendors can take advantage of them with their software. It's basically an API that allows software vendors to query HBA drivers using standard SCSI calls to get information on the device. The commands can also be used to query information from the underlying storage. In order for this to work, the driver you use with your HBA must comply with the specs.
Other methods of auto discovery are to use in-band fabric queries of the SNS name server within the fabric switches or use "out-of-band" methods (usually over TCP/IP) to gather information from the management ports of switches and SAN devices that have standard or advanced MIBS available for them. Those MIBS are sometimes pre-compiled by the ISV management software vendors so they can do inquires and discover the devices it has MIBS for. (An MIB is a management information block, used in standard TCP/IP networks under the SNMP protocol for device management).
As for what can be collected, well, that's up to how complete the MIB is for the device or if the device (like an HBA) uses industry standard drivers that conform to the SNIA and FCIA driver specs. Other devices (like storage arrays) have APIs that can be licensed from the hardware vendor so the management software vendor can not only discover those devices, but also control them (like adding a LUN to a server for instance, or creating a zone in a fabric).
The limitations of auto discovery are that agents usually provide MORE information than standard drivers. An agent sitting on a host server for instance can provide data not only on the SAN storage it sees but also on the internal storage of that server! Agents are usually smarter and can also sometimes provide file system or application level information.
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This was first published in March 2003