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Oracle10g Automated Checkpoint Tuning

Check-pointing is an important Oracle activity which records the highest system change number (SCN,) so that all data blocks less than or equal to the SCN are known to be written out to the data files. If there is a failure and then subsequent cache recovery, only the redo records containing changes at SCN(s) higher than the checkpoint need to be applied during recovery.

As we are aware, instance and crash recovery occur in two steps - cache recovery followed by transaction recovery. During the cache recovery phase, also known as the rolling forward stage, Oracle applies all committed and uncommitted changes in the redo log files to the affected data blocks. The work required for cache recovery processing is proportional to the rate of change to the database and the time between checkpoints.

Fast-start recovery can greatly reduce the mean time to recover (MTTR), with minimal effects on online application performance. Oracle continuously estimates the recovery time and automatically adjusts the check-pointing rate to meet the target recovery time.

Oracle recommends using the fast_start_mttr_target initialization parameter to control the duration of startup after instance failure. With 10g, the Oracle database can now self-tune check-pointing to achieve good recovery times with low impact on normal throughput. You no longer have to set any checkpoint-related parameters.

This method reduces the time required for cache recovery and makes the recovery bounded and predictable by limiting the number of dirty buffers and the number of redo records generated between the most recent redo record and the last checkpoint. Administrators specify a target (bounded) time to complete the cache recovery phase of recovery with the fast_start_mttr_target initialization parameter, and Oracle automatically varies the incremental checkpoint writes to meet that target.

The target_mttr field of v$instance_recovery contains the MTTR target in effect. The estimated_mttr field of v$instance_recovery contains the estimated MTTR should a crash happen right away.

For example,

SQL> SELECT TARGET_MTTR, ESTIMATED_MTTR, CKPT_BLOCK_WRITES FROM V$INSTANCE_RECOVERY;

 

TARGET_MTTR ESTIMATED_MTTR CKPT_BLOCK_WRITES

----------- -------------- -----------------

         37             22            209187

Whenever you set fast_start_mttr_target to a nonzero value, and while MTTR advisory is ON, Oracle Corporation recommends that you disable (set to 0) the following parameters:

 LOG_CHECKPOINT_TIMEOUT
 LOG_CHECKPOINT_INTERVAL
 FAST_START_IO_TARGET

Because these initialization parameters either override fast_start_mttr_target or potentially drive checkpoints more aggressively than fast_start_mttr_target does, they can interfere with the simulation.
 


Get the complete Oracle10g story:

The above text is an excerpt from "Oracle Database 10g New Features: Oracle10g Reference for Advanced Tuning and Administration", by Rampant TechPress.  Written by top Oracle experts, this book has a complete online code deport with ready to use scripts. 

To get the code instantly, click here:

http://www.rampant-books.com/book_2003_2_oracle10g.htm


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