In 8.2, https://www-304.ibm.com/support/docv...id=swg21223978

Memory Changes in DB2 UDB Version 8.2

If you set the instance_memory database manager configuration parameter to AUTOMATIC, the database manager calculates the amount of instance memory needed for the current configuration. The database manager also allocates additional memory for an overflow buffer. This overflow buffer is used to satisfy peak memory requirements for any heap in the instance shared memory region whenever a heap exceeds its configured size.

In the same manner, if you set the database_memory database configuration parameter to AUTOMATIC, the database manager also calculates the amount of database shared memory and allocates additional memory for an overflow buffer.

The recommendation for the new instance memory requirements is to increase the instance_memory parameter by 20%. If you set this parameter to AUTOMATIC, you only need to ensure that your system has available the additional memory requirement.

For database shared memory requirements, the recommendation is to increase the database_memory parameter by 20% plus the value of pckcachesz. If you set the database_memory parameter to AUTOMATIC, you only need to ensure that your system has available the additional memory requirement.



What's the size of an overflow buffer in 9.5?

IBM completely re-wrote the DB2 memory archtecture of DB2 for version 9.5 to use a threaded model. In addition, they also implemented STMM (self tunning memory manager) and other automatic memory features. I am not even sure there is an overflow buffer in 9.5 for database memory, or if there is, then not sure it works in the same way as 8.2.

Overflow buffer is ~20%. Google for ""Exploring Memory in DB2 9.5"