Sizing the redo log files using optimal_logfile_size view.

Question

Regards
I have a specific question regarding logfile size. I have deployed a test database and i was exploring certain aspects with regards to selecting optimal size of redo logs for performance tuning using optimal_logfile_size view from v$instance_recovery. My main goal is to reduce the redo bytes required for instance recovery. Currently i have not been able to optimize the redo log file size. Here are the steps i followed:-
In order to use the advisory from v$instance_recovery i had to set fast_start_mttr_target parameter which is by default not set so i did these steps:-
1)SQL> sho parameter fast_start_mttr_target;
NAME TYPE VALUE
fast_start_mttr_target               integer                           0
2) Setting the fast_start_mttr_target requires nullifying following deferred parameters :-
SQL> show parameter log_checkpoint;
NAME TYPE VALUE
log_checkpoint_interval integer 0
log_checkpoint_timeout integer 1800
log_checkpoints_to_alert boolean FALSE
SQL> select ISSES_MODIFIABLE,ISSYS_MODIFIABLE,ISINSTANCE_MODIFIABLE,ISMODIFIED from v$parameter where name like'log_checkpoint_timeout';
ISSES_MODIFIABL ISSYS_MODIFIABLE ISINSTANCE_MODI ISMODIFIED
FALSE IMMEDIATE TRUE FALSE
SQL> alter system set log_checkpoint_timeout=0 scope=both;
System altered.
SQL> show parameter log_checkpoint_timeout;
NAME TYPE VALUE
log_checkpoint_timeout               integer                           0
3) Now setting fast_start_mttr_target
SQL> select ISSES_MODIFIABLE,ISSYS_MODIFIABLE,ISINSTANCE_MODIFIABLE,ISMODIFIED from v$parameter where name like'fast_start_mttr_target';
ISSES_MODIFIABL ISSYS_MODIFIABLE ISINSTANCE_MODI ISMODIFIED
FALSE IMMEDIATE TRUE FALSE
Setting the fast_mttr_target to 1200 = 20 minutes of checkpoint switching according to Oracle recommendation
Querying the v$instance_recovery view
4) SQL> select ACTUAL_REDO_BLKS,TARGET_REDO_BLKS,TARGET_MTTR,ESTIMATED_MTTR, OPTIMAL_LOGFILE_SIZE,CKPT_BLOCK_WRITES from v$instance_recovery;
ACTUAL_REDO_BLKS TARGET_REDO_BLKS TARGET_MTTR ESTIMATED_MTTR OPTIMAL_LOGFILE_SIZE CKPT_BLOCK_WRITES
276 165888 *93* 59 361 16040
Here Target Mttr was 93 so i set the fast_mttr_target to 120
SQL> alter system set fast_start_mttr_target=120 scope=both;
System altered.
Now the logfile size suggested by v$instance_recovery is 290 Mb
SQL> select ACTUAL_REDO_BLKS,TARGET_REDO_BLKS,TARGET_MTTR,ESTIMATED_MTTR, OPTIMAL_LOGFILE_SIZE,CKPT_BLOCK_WRITES from v$instance_recovery;
ACTUAL_REDO_BLKS TARGET_REDO_BLKS TARGET_MTTR ESTIMATED_MTTR OPTIMAL_LOGFILE_SIZE CKPT_BLOCK_WRITES
59 165888 93 59 290 16080
After altering the logfile size to 290 as show below by v$log view :-
SQL> select GROUP#,THREAD#,SEQUENCE#,BYTES from v$log;
GROUP# THREAD# SEQUENCE# BYTES
1 1 24 304087040
2 1 0 304087040
3 1 0 304087040
4 1 0 304087040
5 ) After altering the size i have observed the anomaly as redo log blocks to be applied for recovery has increased from *59 to 696* also now v$instance_recovery view is now suggesting the logfile size of *276 mb*. Have i misunderstood something
SQL> select ACTUAL_REDO_BLKS,TARGET_REDO_BLKS,TARGET_MTTR,ESTIMATED_MTTR, OPTIMAL_LOGFILE_SIZE,CKPT_BLOCK_WRITES from v$instance_recovery;
ACTUAL_REDO_BLKS TARGET_REDO_BLKS TARGET_MTTR ESTIMATED_MTTR OPTIMAL_LOGFILE_SIZE CKPT_BLOCK_WRITES
*696* 646947 120 59 *276* 18474
Please clarify the above output i am unable to optimize the logfile size and have not been able to achieve the goal of reducing the redo log blocks to be applied for recovery, any help is appreciated in this regard.

Answer 1

sunny_123 wrote:
Sir oracle says that fast_start_mttr target can be set to 3600 = 1hour. As suggested by following oracle document
http://docs.oracle.com/cd/B10500_01/server.920/a96533/instreco.htm
I set mine value to 1200 = 20 minutes. Later i adjusted it to 120=2 minutes as Target_mttr suggested it to be around 100 (if fast_mttr_target value is too high or too low effective value is contained in target_mttr of v$instance_recovery)Just to add, you are reading the documentation of 9.2 and a lot has changed since then. For example, in 9.2 the parameter FSMTTR was introduced and explicitly required to be set and monitored by the DBA for teh additional checkpoint writes which might get caused by it. Since 10g onwards this parameter has been made automatically maintained by Oracle. Also it's been long that 9i has been desupported followed by 10g so it's better that you start reading the latest documentation of 11g and if not that, at least of 10.2.
Aman....