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Don Burleson Blog 







Oracle 11g hidden parameter tips

Oracle Database Tips by Donald BurlesonNovember 20,  2015

Starting in Oracle 11g, we have a heap of new hidden parameters, many of which control important new features such as the adaptive cursor sharing feature.

Key 11g Hidden Parameters for Oracle

With the Oracle release, there are many new hidden parameters that maintain a great deal of the functionality for the myriad of new features that come with as shown below. For example, the following hidden parameters are new to Oracle and support many new features:

  • diag_adr_enabled
  • diag_adr_auto_purge
  • prop_old_enabled
  • diag_hm_rc_enabled

New Features and 11g Hidden Parameters

Every new feature within Oracle has a hidden and undocumented parameter associated with it that contains the critical settings for the internal operations of the functionality within the Oracle database. The Oracle Automatic Diagnostic Repository (ADR) contains the following key hidden parameters:

  • _diag_adr_test_param                 
  • _diag_adr_enabled
  • _diag_adr_auto_purge          

The diag_adr_test_param is a hidden parameter that serves as a testing parameter to verify operation of the ADR diagnostic features.

Another key hidden parameter for the ADR new feature with Oracle is diag_adr_enabled. This parameter either enables or disables the operation of the ADR feature.

Finally, there is the diag_adr_auto_purge parameter for Oracle which allows one to enable or disable the ADR Memory Monitorbackground process behavior with respect to the ADR operations.

Oracle 11g Hidden Parameters for Data Recovery Advisor (DRA)

The Oracle Data Recovery Advisor (DRA) is an exciting new feature that is new to for backup and recovery functionality. The following hidden parameters perform the behind the scenes add-on functions as part of the DRA with Oracle :

  • dra_enable_offline_dictionary
  • dra_bmr_number_threshold
  • dra_bmr_percent_threshold

The hidden parameter dra_enable_offline_dictionaryenables the periodic creation for offline dictionary objects associated with the Data Recovery Advisor. The other two hidden parameters pertain to block media recovery(BMR) tasks as performed with database backup and recovery operations by the Data Recovery Advisor within Oracle. As for the hidden parameter dra_bmr_number_threshold, this parameter relates to the maximum amount of block media recovery tasks that can be performed on a file during recovery. The other hidden parameter for the Data Recovery Advisor, dra_bmr_percent_threshold, measures the largest percentage of blocks in a file during recovery tasks performed by the DRA for Oracle.      

Hang Detection Parameters for the Health Monitor (HM) System

As part of the self-healing and tuning new features for, Oracle has provided the health monitor (HM) suite of tools and hidden parameters listed next.

  • _hang_detection
  • _hang_resolution
  • _hm_analysis_output_disk
  • _hm_analysis_oradebug_node_dump_level
  • _hm_analysis_oradebug_sys_dump_level
  • _diag_hm_rc_enabled
  • _diag_hm_tc_enabled

The hidden parameter hang_detection provides for an interval to detect hang conditions as part of the health monitor tasks. For hang resolution, there is the hidden parameter hang_resolution.

The hm_analysis_output_disk is a hidden parameter which generates a return code of TRUE if the hang manager outputs hangs within the Oracle database. For Oradebug dump activity, there are also two hidden parameters. The first hidden parameter that coordinates Oradebug node dump level output for hang manager analysis is hm_analysis_oradebug_node_dump_level. 

Another hidden parameter for related tasks with conjunction to Oradebug system level traces is the hm_analysis_oradebug_sys_dump_level parameter which allows the hang manager to output system state level information with the Oradebug utility.

For the health monitor (HM) checks performed by the Oracle health checkers, there are a couple of new undocumented database parameters: diag_hm_rc_enabled and diag_hm_tc_enabled. The parameter diag_hm_rc_enabled performs the task of either enabling or disabling the health monitor checks for diagnosis of database health conditions. There is also a partner hidden parameter called diag_hm_tc_enabled that allows one to either enable or disable the test or dummy checks performed by the health monitor system within Oracle.

Note:  Hidden and undocumented parameters are very powerful and should ONLY be set with the express consent of Oracle technical support. 
Here is a list of the hidden parameters in Oracle11g:
_ksb_restart_policy_times='240′ # internal update to set default



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