Oracle Database Administration: The Essential Refe

LGWR—the Log Writer

The Log Writer process is responsible for writing the contents of the redo log buffer to the log file on disk, and for management of the log buffer. Redo log data is always written first to a buffer in memory, then written to disk by the LGWR process when:

• LGWR is not active for three seconds.

• The redo log buffer becomes one-third full.

• The DBWR process writes dirty buffers to disk.

The LGWR process is also responsible for the following:

• Writing commit records to the log file when a transaction is committed. Multiple commits (known as group commits ) may be written in a single operation when database activity is high.

• Recording a system change number (SCN) to each committed transaction, for use in recovery operations when running Parallel Server.

• Updating the headers of all datafiles when a checkpoint occurs, unless the CKPT process (described under Section 10.2.2.1, later in this chapter) is active.