DFS houses fileset and Backup System information in two administrative databases: The Fileset Location Database (FLDB) stores information about the locations of filesets; the Backup Database records information about backups and tapes. To maintain file system reliability and availability, the two databases are replicated on multiple server machines. If any of the machines housing a database becomes unavailable, the database is still available from other machines.
Administrators can use the multihomed server capabilities in DFS to provide the most efficient network access from DFS clients to FLDB server machines. Each machine can have up to four IP addresses, providing network connections to the subnetworks or networks that have the highest concentration of DFS clients. Should a particular FLDB machine connection become unavailable, the Cache Managers on the various DFS clients then reference their lists of server preferences to connect to the next preferred address for an FLDB machine. By default, the preference values are chosen to make reasonable decisions about the order in which servers are accessed. For example, the default preference values bias a Cache Manager to first access FLDB machines within its same subnetwork before contacting machines in other subnetworks.
To synchronize the information in the databases, DFS uses a library of utilities called Ubik. Ubik is a synchronization mechanism that distributes changes to fileset and backup information to all copies of the appropriate database. Administrators need to be aware of which machines store copies of a database only when the machines are configured. Once the machines are configured, administrators, like users, never need to know which server machines store copies of a database; they merely make changes to information in a database, and Ubik coordinates the updating of the information to all sites at which the database is replicated. The distribution across the database sites is automatic and almost instantaneous.