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15.7. 32-Bit Memory Model Considerations

If you install DB2 on a 32-bit operating system, there are limitations to the maximum size of a shared memory segment that a process can allocate. Since each database has all of its shared memory allocated in the same segment, there is a limit on the total amount of shared memory that can be allocated per database.

In 32-bit AIX, there are a maximum of sixteen 256MB shared memory segments that can be addressed by a process. Of these sixteen segments, only seven can be used by DB2 for shared memory. By default, DB2 reserves one of these seven segments for optimizing memory-mapped I/O. If you have multiple logical partitions on the same server, DB2 also uses one of these seven segments for inter-partition (FCM) communications. DB2 lets you maximize the amount of memory available for the buffer pools and other shared memory components by allowing you to disable memory-mapped I/O and the FCM use of shared memory for inter-partition communication.

NOTE

On AIX, memory-mapped I/O uses I-nodes to optimize access to files rather than tracking file handles.

To disable memory-mapped I/O, you can set the following DB2 registry variables to NO using the

db2set command.

• DB2_MMAP_READ

• DB2_MMAP_WRITE

To force the FCM communications to use the network interconnect instead of shared memory for inter-partition communication between logical partitions, you can set the following DB2 registry variable to NO using the

db2set command.

• DB2_FORCE_FCM_BP

The maximum addressable amount of shared memory for the 32-bit version of DB2 varies depending on the operating system. Table 15.1 lists the memory limits.