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Oracle Instance

An Oracle instance consists of the Oracle software and the memory structures that interact with the database. It plays a key role in managing and processing the data stored in an Oracle database.

1. Instance Overview

  • An Oracle instance is the combination of memory structures and background processes that Oracle uses to manage a database.
  • When an Oracle instance starts, it uses initialization parameters to allocate memory and start background processes.
  • Each instance is associated with one database but multiple instances can manage a single database in a Real Application Cluster (RAC) configuration.

2. Components of an Oracle Instance

An Oracle instance consists of the following major components:

a. Memory Structures

Oracle instance includes several memory structures that help in storing data and caching information:

  • System Global Area (SGA): A shared memory area that contains data and control information for the instance.

    • Database Buffer Cache: Stores data blocks that have been recently accessed.
    • Redo Log Buffer: Stores changes made to the database before writing them to the redo logs.
    • Shared Pool: Caches SQL statements, parsed SQL, and execution plans.
    • Large Pool, Java Pool, Streams Pool: Used for specific tasks like parallel query execution, Java code, etc.

  • Program Global Area (PGA): A memory area used by a single Oracle process, which contains session-specific information.

b. Background Processes

Oracle uses several background processes that help manage database operations:

  • DBWn (Database Writer): Writes modified data blocks from the database buffer cache to the data files.
  • LGWR (Log Writer): Writes redo log entries from the redo log buffer to the redo log files.
  • CKPT (Checkpoint): Signals database checkpoints, updating control and data files.
  • SMON (System Monitor): Performs crash recovery when needed.
  • PMON (Process Monitor): Cleans up after failed processes and frees resources.
  • ARCn (Archiver): Copies redo log files to the archive destination in archiving mode.

3. Instance Startup and Shutdown

  • Startup:

    1. Nomount: The instance is started and memory structures are allocated, but the database is not yet associated with the instance.
    2. Mount: The database control files are read, and the database is associated with the instance, but the database is not yet open for use.
    3. Open: The data files and redo log files are opened, making the database available for use.

  • Shutdown: Oracle instance shutdown can be done in different modes:

    • NORMAL: Waits for all sessions to disconnect and then shuts down.
    • IMMEDIATE: Rolls back active transactions and disconnects users.
    • ABORT: Instantly stops the instance without rolling back transactions.

4. Instance vs Database

  • An Oracle database is a collection of physical files (data files, redo log files, control files).
  • An Oracle instance is the running Oracle program that provides access to the data in the database.

5. Real Application Clusters (RAC)

  • In an Oracle RAC environment, multiple instances run on different servers but share the same physical database, providing high availability and scalability.

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