SGA_MAX_SIZE != SGA_TARGET when?

Question

Just curious... under what circumstances, when using ASMM, would I not want SGA_TARGET to equal SGA_MAX_TARGET?
Bob

Answer 1

Just to clarify things up,
Oracle will allocate SGA size equal to SGA_MAX_SIZE from system initially.
SGA_TARGE can be smaller than SGA_MAX_SIZE because they are buffer caches, such as KEEP, RECYCLE, and other block sizes, not affected by SGA_TARGET.
See the result below,
SYS@etest> show sga
Total System Global Area  264241152 bytes
Fixed Size                  1301408 bytes
Variable Size             216540256 bytes
Database Buffers           46137344 bytes
Redo Buffers                 262144 bytes
SYS@etest> show parameter sga
NAME                                 TYPE        VALUE
lock_sga                             boolean     FALSE
pre_page_sga                         boolean     TRUE
sga_max_size                         big integer 252M
sga_target                           big integer 200MOracle SGA size is 264241152/1024/1024=252M equal to SGA_MAX_SIZE while SGA_TARGET is 200M
Since I don't have other buffers set, Where the 50M goes ?
SYS@etest> select * from v$sgainfo
  2  /
NAME                                  BYTES RES
Fixed SGA Size                      1301408 No
Redo Buffers                         262144 No
Buffer Cache Size                  46137344 Yes
Shared Pool Size                  150994944 Yes
Large Pool Size                     4194304 Yes
Java Pool Size                      4194304 Yes
Streams Pool Size                         0 Yes
Granule Size                        4194304 No
Maximum SGA Size                  264241152 No
Startup overhead in Shared Pool    37748736 No
Free SGA Memory Available 54525952
11 rows selected.It went to Free SGA Memory Available. Hence you can dynamically increase SGA_TARGET without reboot instance.