Oracle System Identifier (SID)
The System identifier (SID) is a unique name for an Oracle database instance on a specific host. On UNIX and Linux, Oracle Database uses the SID and Oracle home values to create a key to shared memory. Also, the SID is used by default to locate the parameter file, which is used to locate relevant files such as the database control files. On most platforms, the ORACLE_SID environment variable sets the SID, whereas the ORACLE_HOME variable sets the Oracle home. When connecting to an instance, clients can specify the SID in an Oracle Net connection or use a net service name. Oracle Database converts a service name into an ORACLE_HOME and ORACLE_SID.
Oracle Database uses checkpoints to achieve the following goals:
■Reduce the time required for recovery in case of an instance or media failure
■Ensure that dirty buffers in the buffer cache are written to disk regularly
■Ensure that all committed data is written to disk during a consistent shutdown
The checkpoint process (CKPT) is responsible for writing checkpoints to the data file headers and control file. Checkpoints occur in a variety of situations. For example, Oracle Database uses the following types of checkpoints:
The database writes to disk all buffers modified by redo in a specific thread before a certain target. The set of thread checkpoints on all instances in a database is a database checkpoint. Thread checkpoints occur in the following situations:
– Consistent database shutdown
– ALTER SYSTEM CHECKPOINT statement
– Online redo log switch
– ALTER DATABASE BEGIN BACKUP statement
■Table space and data file checkpoints
The database writes to disk all buffers modified by redo before a specific target. A table space checkpoint is a set of data file checkpoints, one for each data file in the table space. These checkpoints occur in a variety of situations, including making a table space read-only or taking it offline normal, shrinking a data file, or executing ALTER TABLE SPACE BEGIN BACKUP.
An incremental checkpoint is a type of thread checkpoint partly intended to avoid writing large numbers of blocks at online redo log switches. DBW n checks at least every three seconds to determine whether it has work to do. When DBW n writes