When you set up an Xsan SAN, you assign at least one computer to act as the
metadata controller. The controller manages volume metadata, maintains a file system
journal, and controls concurrent access to files. Metadata includes such information as
where files are stored and what portions of available storage are allocated to new files.
To guarantee volume availability, a SAN should include more than one metadata
controller, as shown in the illustration on page 14. If the primary controller fails, the
standby controller takes over.