The sga_max_size parameter is used to indicate the maximum overall size of the SGA for the lifetime of the instance. You can dynamically alter the size of the buffer cache, shared pool, large pool, streams pool or the java pool but only to the extent that the sum of these memory areas and the size of other components like fixed SGA, variable SGA, log buffer, keep and recycle buffer caches, if configured and Non-Standard block size buffer caches, if configured, do not exceed the value specified by SGA_MAX_SIZE.
The sga_max_size default is the total size of the configured SGA at instance startup. If sga_max_size is set to a smaller value than the amount of memory initially allocated at instance startup, sga_max_size defaults to the total amount of memory initally allocated. This parameter cannot be dynamically changed. Therefore, make sure it is set correctly in case you will ever want to increase overall SGA memory use. Note that this parameter will cause Oracle to reserve memory of an amount of sga_max_size on most operating systems. Use caution when setting this parameter to avoid causing swapping or paging.
The change in the amount of physical memory consumed when SGA_TARGET is modified depends on the operating system. On some UNIX platforms that do not support dynamic shared memory, the physical memory in use by the SGA is equal to the value of the SGA_MAX_SIZE parameter. On such platforms, there is no real benefit in setting SGA_TARGET to a value smaller than SGA_MAX_SIZE. Therefore, setting SGA_MAX_SIZE on those platforms is not recommended.
On other platforms, such as Solaris and Windows, the physical memory consumed by the SGA is equal to the value of SGA_TARGET.
