How to Switch Oracle 11g Between ASMM and AMM – Step-by-Step

1. Experiment Environment

We use Oracle Database 11.2.0.3 on a Linux host, initially running with ASMM (Automatic Shared Memory Management). The version is verified with select * from v$version; and the current memory settings show memory_target=0 and memory_max_target=0, indicating AMM is disabled.

2. Switching from ASMM to AMM

In 11g, setting memory_target and memory_max_target to non‑zero values enables AMM. Before changing parameters, we back up the spfile with create pfile from spfile;. Then we execute the following ALTER SYSTEM commands (scope=spfile):

alter system set memory_max_target=360m scope=spfile;
alter system set memory_target=360m scope=spfile;
alter system set sga_target=0m scope=spfile;
alter system set sga_max_size=0 scope=spfile;
alter system set pga_aggregate_target=0 scope=spfile;

After restarting the instance, show parameter target confirms that both MEMORY_TARGET and MEMORY_MAX_TARGET are 360M while SGA_TARGET and PGA_AGGREGATE_TARGET are 0, meaning AMM is active and the shared memory segment becomes a virtual segment. The ipcs -m output shows small 4 KB segments.

3. Switching back from AMM to ASMM

To disable AMM completely, set both MEMORY_TARGET and MEMORY_MAX_TARGET to 0, then assign explicit values to SGA and PGA:

alter system set memory_max_target=0 scope=spfile;
alter system set memory_target=0 scope=spfile;
alter system set pga_aggregate_target=100m scope=spfile;
alter system set sga_target=260m scope=spfile;
alter system set sga_max_size=260m scope=spfile;

A subsequent startup fails with ORA‑00843 and ORA‑00849 because the SGA_TARGET exceeds the zero MEMORY_MAX_TARGET. The fix is to recreate the spfile from a pfile that omits the explicit MEMORY_TARGET entries. After creating a pfile ( create pfile from spfile;), editing out the lines *.memory_target=0 and *.memory_max_target=0, and then creating a new spfile ( create spfile from pfile;), the database starts successfully with ASMM enabled.

4. Conclusion

AMM and ASMM are essential Oracle memory‑management tools; AMM offers convenient automatic tuning, while ASMM provides real shared memory segments. In certain scenarios, such as when using HugePages, switching back to ASMM may be necessary.