AMM question

Question

Hi,
I have read this article: http://oracle-online-help.blogspot.ro/2008/03/memory-parameters-in-11g-memorytarget.html
And i see:
>
If MEMORY_TARGET is set to non zero value:
SGA_TARGET, SGA_MAX_SIZE and PGA_AGGREGATE_TARGET are set to 0, 60% of memory mentioned in MEMORY_TARGET is allocated to SGA and rest 40% is kept for PGA.
>
So, I have set:
alter system set sga_target=0 scope=spfile;
alter system set pga_aggregate_target=0 scope=spfile;
alter system set sga_max_size=0 scope=spfile;
alter system set memory_max_target=512M scope=spfile;
alter system set memory_target=300M scope=spfile;
, and then bounced the instance.
After startup, I see:
SQL> startup
ORACLE instance started.
Total System Global Area  272027648 bytes
Fixed Size                  1384012 bytes
Variable Size             100663732 bytes
Database Buffers          163577856 bytes
Redo Buffers                6402048 bytes
Database mounted.
Database opened.
SQL>But Total System Global Area should't be in that case 60% of memory mentioned in MEMORY_TARGET? Memory mentioned in MEMORY_TARGET was 300M, and 60% of 300M is 180M, which is not 272027648 bytes.
I just want to use automatic memory management, so I've set the other parameters above to 0.
My instance is only on my local machine, so for my own personal use.
Please clarify.
Thanks.

Answer 1

What platform and RDBMS version are you using?
The SGA_MAX_SIZE is the upper boundary limit on your SGA. One one of my test systems, I see that it is the same value as my AMM settings:
memory_max_target               big integer 600M
memory_target                      big integer 600M
sga_max_size                         big integer 600MLet's see what happens when I set SGA_MAX_SIZE to zero:
SQL> alter system set sga_max_size=0 scope=spfile;
System altered.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.
Total System Global Area  409251840 bytes
Fixed Size                  2228744 bytes
Variable Size             251661816 bytes
Database Buffers          146800640 bytes
Redo Buffers                8560640 bytes
Database mounted.
Database opened.
SQL> show parameter sga_max;
NAME                                 TYPE        VALUE
sga_max_size                         big integer 392MSo, you can see it took about 60%, as expected. What you're seeing is likely the absolute minimum SGA that Oracle requires. If you're able to allocate more memory, you're likely to see the 60/40 percent behavior you're expecting.