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Don Burleson Blog 







ORA-00845: MEMORY_TARGET not supported on this system tips

Oracle Database Tips by Donald BurlesonDecember 16,  2015 - Updated March 20, 2016

Question:  I was changing memory_max_target when I got this ORA-00845 error:

ORA-00845: MEMORY_TARGET not supported on this system

Answer:  You will get a ORA-00845 error if your shared memory file system is not big enough to accommodate memory_target and memory_max_target. There are two possible ways to remove the ORA-00845 error:

- Reduce the value for the memory_target parameter

- Increase the temporary memory allocation on the server.  This should fix the ORA-00845 error:

# umount tmpfs
# mount -t tmpfs shmfs -o size=1500m /dev/shm

The oerr message ORA-00845 addresses this error:

ORA-00845: MEMORY_TARGET not supported on this system

Cause: The MEMORY_TARGET parameter was not supported on this operating system or /dev/shm was not sized correctly on Linux.

Action: Refer to documentation for a list of supported operating systems. Or, size /dev/shm to be at least the SGA_MAX_SIZE on each Oracle instance running on the system.

Oracle says that if you want to use Linux HugePages make sure that both MEMORY_TARGET / MEMORY_MAX_TARGET initialization parameters are disabled for the database instance. 

AMM is a memory-morphing tool that adjusts the sizes of the SGA pool reactively, as workloads change.  It works well on tiny systems, but for large databases, the DBA can perform the SGA pool size adjustment proactively, before a workload change cripples performance.

Oracle does NOT recommend using Automatic Memory Management (AMM, e.g. setting memory_target) with Linux hugepages.  See MOSC note 749851.1 "HugePages and Oracle Database 11g Automatic Memory Management (AMM) on Linux" for important details.   In sum, AMM is not compatible with Linux Hugepages. 

Also, beware that AMM re-size operations can cripple Oracle performance in some cases.

For mission-critical databases, I recommend building intelligent SGA monitors that predict workload changes before they occur and change the SGA just-in-time.  For complete details, see my book Oracle Tuning: The Definitive Reference.

Note:  When using AMM (by setting memory_target, and/or sga_target, the values for the "traditional" pool parameters (db_cache_size, shared_pool_size, &c) are not ignored.  Rather, they will specify the minimum size that Oracle will always maintain for each sub-area in the SGA.




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