由 macro 发布于: 2001-03-02 12:51

A log switch occurs when the LGWR stops writing to an online redo log group and starts writing to another.

Log switches and checkpoints are events that happen automatically ; for example, when the current lonline redo log file group is filled,but log switches and checkpoints can be forced.

SQL> ALTER SYSTEM SWITCH LOGFILE;

SQL> ALTER SYSTEM CHECKPOINT;


When the database uses large online redo log files,you can set additional database checkpoints by setting the initialization parameters:

1.LOG_CHECKPOINT_INTERVAL
For version prior to release 8.1 ,a checkpoint is initiated as soon as the LGWR writes the number of blocks specified by the this parameter.

It's value is specified in operation system blocks and not in oracle database blocks.

Regardless of this value , a checkpoint always occurs when switching from one online redo log file to another.

If the value exceeds the actual online redo log file size,checkpoints occur only when switching log .

Note that specifying a value of 0 for the interval might cause checkpoints to be initiated very frequently , because a new request will be started even if a single redo log buffer has been written since the last request was initiated.

With release 8.1,when LOG_CHECKPOION_INTERVAL is specified, the target for checkpoion position cannot lag the end of the log more than the number of redo log blocks specified by this parameter.This ensures that no more than a fixed number of redo blocks will need to be read during instance recovery.

2.LOG_CHECKPOINT_TIMEOUT
For versions prior to release 8.1 , the value of this initialization parameter specifies the maxinum amount of time before another checkpoint occurs.This value is specified in seconds.The time begins at the start of the previous checkpoint ,and then a checkpoint occurs after the amount of time specified by this parameter.

Specifing a value of 0 for the timeout disable time-based checkpoints.

With release 8.1,when this parameter is specified,it sets the target for checkpoint position to a location in the log file where the end of the log was this many seconds ago .This ensures that no more than the specified number of secojnds' worth of redo blocks needs to be read during recovery.

3.FAST_START_IO_TARGET
This parameter improves the performance of crash and instance recovery.The smaller the value of this parameter,the better the recovery performance,because fewer blocks need to be recovered.When the parameter is set,the DBWn writes dirty buffers out more aggressively.

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Macro Zeng

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