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<HEAD><TITLE>论坛精华 >> Oracle 专栏 >> A 7-3 LGWR,Log Switches, and Checkpoints</title>
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>> A 7-3 LGWR,Log Switches, and Checkpoints [<a href=http://www.jsp001.com/forum/showthread.php?goto=newpost&threadid=554>查看别人的评论</a>]<br>
<hr><p>由 macro 发布于: 2001-03-01 12:53</p><p><img src="images/icons/icon6.gif" alt="Cool" border=0> </p><p>Redo Log Buffer and the LGWR Background Process<br><br>The oracle server sequentially records all changes made to the database in the redo log buffer.The redo log buffer is used in <font color=red>a circular manner</font>.The redo entries are written to <font color=red>one of the online redo log groups</font> called the current online redo log group by the LGWR process under the following situations:<br><br>1. When a transaction commits<br>2. When the redo log buffer becomes one-third full<br>3. When there is more than a megabyte of changed records in the redo log buffer.<br>4. When timeout occurs(every 3 seconds)<br>5. Before the DBWn writes modified blocks in the database buffer cache to the data files.<br><br>Log Switches <br><br>LGWR writes to the online redo log files sequentially - that is , when the current online redo log group is filled , LGWR begins writing to the next group ,when the last available online redo log file is filled, LGWR returns to the first online redo log group and starts writing again .<br><br>The database administrator can also force log switches.Each time a log switch occurs and LGWR begins writing to a new log group, the oracle server assigns a number known as the log sequence number to identify the set of redo entries.<br><br>When a log switch occurs, and event called a checkpoint is initiated.<br><br>A log switch is the event during which LGWR stops writing to one online redo log group and starts writing to another.<br><br></p><hr><p>由 macro 发布于: 2001-03-01 13:04</p><p> </p><p>During a checkpoints:<br><br>1. A number of dirty database buffers covered by the log being checkpointed are written to the data files by DBWn . The number of buffers being written by DBWn is determined by the parameter FAST_START_IO_TARGET , if specifed.<br><br>2.The checkpoint background process CKPT updates the headers of all data files and control files to reflect that it has completed successfully.<br><br>Checkpoints can occur for all data files in the database or for only specific data files.<br><br>A checkpoint occurs , for example, in the following situations:<br><br>1.At every log switch<br>2.when an instance has been shut down with the normal ,transactional,or immediate option<br>3.When forced by setting the initialization parameters: LOG_CHECKPOINT_INTERVAL,LOG_CHECKPOINT_TIMEOUT,and FAST_START_IO_TARGET<br>4.When manually requested by the database administrator<br><br><br>Information about each checkpoint is recorded in the ALERT file if the initialization parameter LOG_CHECKPOINT_TO_ALERT is set to TURE. <font color=red>The default value of FALSE</font> for this parameter does not log checkpoint.</p><hr><p>由 macro 发布于: 2001-03-01 13:05</p><p> </p><p>The FAST_START_IO_TARGET parmeter has been added in release 8.1</p></td>
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