Whenever an instance starts up, it verifies access to all online datafiles. The first instance to start up in a parallel server must verify access to all online files so that it can determine if media recovery is required. Additional instances can operate without access to all of the online datafiles, but any attempt to use an unverified file fails and a message is generated. When an instance adds a datafile or brings an offline datafile online, all instances verify access to the file. If an instance adds a new datafile on a disk that other instances cannot access, verification fails, but the instances continue running.
Verification can also fail if instances access different copies of the same datafile.
Oracle cannot recover from instance failure or media failure unless the instance that performs recovery can verify access to all of the required online datafiles. Oracle automatically maps absolute file numbers to relative file numbers. Use of a parallel server does not affect these values. For more information about relative file numbers, see Oracle8 Server Concepts . In a parallel server, each instance writes to its own set of online redo log files.