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_memory_imm_mode_without_autosga - no really ! don't resize my sga ! I mean it !

Wow - I need a mental note for this one ! (hence the blogpost)

So you've disable Automatic Memory Management (AMM/ASMM) on your new 11.2 instance - memory_target and sga_target are set to zero ... because ... the dba knows best!
You've configure large pages - locked the sga in memory - manual configured buffercache etc - you've got the luxury of finally having a machine with gigs and gigs of memory.

And then one day you find out that your buffercache is 256Mb in size, the large pool is now 20Gb (coming from 1Gb) and the alert file is screaming ORA-4031, ORA-4031 !!!

How did that happen !!!!!! We didn't use automatic memory management ! You scream and yell - who touched the instance parameters..... no fool would set a large pool of 20Gb and a buffer cache of 256Mb manually.
Looking in the audit logs reveals nobody touched the instance ... it seems AMM/ASMM *is* at play - how can that be if we disabled it ?

You open a SR with support and everything becomes clear now :
- we just ignore your settings and do AMM anyway - if you don't want that set this underscore parameter :
alter system set "_memory_imm_mode_without_autosga"=FALSE scope=both;

Quote - "This is expected behavior in 11.2 for immediate memory allocation requests, which added this as a new feature when automatic memory management was disabled."
Stupid me !
note on metalink "SGA Re-Sizes Occurring Despite AMM/ASMM Being Disabled (MEMORY_TARGET/SGA_TARGET=0) [ID 1269139.1]"

See DUDE primer for info

Product is discontinued.