Oracle startup mount exclusive作用

If your Oracle server allows multiple instances to access a single database concurrently (Oracle Parallel Server option), choose whether to mount the database exclusively or in parallel. For example, to open in parallel mode you can issue:

STARTUP OPEN sales PFILE=initsale.ora PARALLEL;

Multiple instances can now access the database.

If you specify EXCLUSIVE (the default), then the database can only be mounted and opened by the current instance. The following statement starts an instance, mounts and opens the database named sales in exclusive mode, and restricts access to administrative personnel.

STARTUP OPEN sales PFILE=initsale.ora EXCLUSIVE RESTRICT;

Taken from: http://download.oracle.com/docs/cd/A87860_01/doc/server.817/a76956/start.htm#6155