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DBA - SGA: Redo Log Buffer

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• DBA - SGA: Redo Log Buffer
  • Redo Log Buffer

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Redo Log Buffer

• redo log buffer

  • a circular buffer in the SGA that stores redo entries describing changes made to the database.

• redo record

  • a data structure that contains the information necessary to reconstruct, or redo, changes made to the database by DML or DDL operations.

• Database recovery applies redo entries to data files to reconstruct lost changes.

• The database processes copy redo entries from the user memory space to the `redo log buffer` in the SGA.

  • The redo entries take up continuous, sequential space in the buffer.

• The background process log writer process (LGWR) writes the redo log buffer to the active online redo log group on disk.

Process	From	To
DB Process	user memory space	redo log buffer
LGWR	redo log buffer	onlinve redo log group

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• DBW vs LGWR对比

  • DBW performs scattered writes of data blocks to disk.
  • LGWR writes redo sequentially to disk

• Scattered writes tend to be much slower than sequential writes.

• parameter LOG_BUFFER:

  • specifies the amount of memory that Oracle Database uses when buffering redo entries.
  • Unlike other SGA components, the redo log buffer and fixed SGA buffer do not divide memory into granules.

SHOW PARAMETER LOG_BUFFER;

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