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BEWARE:  Oracle 11g New Feature Causes AMM to Restart

Expert Oracle Database Tips by Donald BurlesonMarch 14, 2015

 

Beware, following new upgrades to 11gR2, you can find that AMM has reappeared, even if you think that you have disabled it.   Also see these other 11g release 2 gotchas.

Oracle Automatic Memory Management (AMM) was introduced in Oracle 10g for small databases where a one-sized-fits-all approach was used in small ?hands-off? databases that did not have a DBA to intelligently allocate the SGA pools.  

When a small system has a too small RAM region for the SGA, Oracle AMM will cause a performance problem with RAM thrashing and it tries to readjust all of the RAM regions.  

In all large mission-critical databases, the DBA will disable AMM because an intelligent human will always do a better job than a simple software solution, especially when you need to intelligently reconfigure the SGA in anticipation of a changing workload.

The following references will be helpful in researching the behavior of AMM:

Disabling AMM in Oracle 11g Release 2  

You can enable and disable AMM by changing the value of these parameters to non-zero settings:

  • sga_target

  • sga_max_size

  • pga_aggregate_target

  • sga_max_size

  • db_cache_size

  • shared_pool_size

  • java_pool_size

Unfortunately a "new feature" in 11gR2 turns AMM on unless a hidden parameter called _memory_imm_mode_without_autosga is set to FALSE.  

Important 11gR2 Note:  Even if AMM has been disabled in Oracle 10g, AMM may reappear in Oracle 11g unless the following parameter is set:

alter system set "_memory_imm_mode_without_autosga"=FALSE scope=both;  

There is a MOSC note about AMM in 11gR2 that notes that this regression back to AMM is the expected behavior in 11.2 for immediate memory allocation requests and that Oracle added this behavior as a 11gR2 new feature when automatic memory management was disabled.

MOSC Notes:

  • Bug 7272646 - This bug appears whenever memory_target > 3g, causing the instance to crash at database open time.  If you set memory_target = 0 them AMM is turned off and we have no issues.  If you must use automatic memory management (setting memory_target), download and apply patch 7272646 from MOSC.

  • Note 169706.1 - Some guru's say that the server shared memory (/dev/shm) needs to be at least 2g greater than memory_target else the dynamic memory allocation becomes inappropriate and can crash the server.

  • ORA-27103: internal error
    Linux-x86_64 Error: 2: No such file or directory
    Additional information: -1
    Additional information: 1
    MMAN (ospid: 12211): terminating the instance due to error 27103

  •  Note 1269139.1: AMM regression is , titled:  'sGA Re-Sizes Occurring Despite AMM/ASMM Being Disabled(MEMORY_TARGET/SGA_TARGET=0)?

 
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