New default limit on UBM size in Domino 8

On systems with more than 2 GB of RAM installed, Lotus® Domino® 6 server and Domino 7 can calculate the maximum size of the Unified Buffer Manager (UBM) to an inappropriately large value of 1.5 GB, particularly on Unix systems. Unless an administrator knows to lower this maximum limit with an INI parameter, an increased UBM size can result in high overall shared memory usage.

Depending on the platform, this increased shared memory usage can cause a low memory condition or crash, for instance on AIX, which enforces a lower limit on shared memory usage by default.

Prior to Domino 8, the algorithm that Domino uses for setting the maximum size for the UBM has remained unchanged since Notes Release 4. Domino scales the upper limit of the UBM based on physical RAM on the system, taking roughly 3/8th the amount of RAM up to 4 GB. On larger systems, this can result in a UBM of 1.5 GB in size. Information in the field indicates that this value is overly large for Domino 32-bit servers, even on large systems.

Domino 8 and higher versions now implement a lower, more effective limit to the size of the UBM in order to accommodate larger systems. Testing and performance data from the field indicates that a UBM limit of 512 MB is the optimum size limit on most platforms, allowing Domino to realize efficient disk I/O when accessing database structures on disk. Values higher than this have been found to result in increased memory usage without significant performance improvements.

The Domino 8 server enforces this limit by default with no additional notes.ini file parameters required. Domino will still honor the setting NSF_BUFFER_POOL_SIZE_MB (as well as related parameters) on upgraded servers, which can be used to configure the UBM to be larger than 512 MB if necessary. The default limits depend on platform, which are listed in the table below:

Operating System	Domino 8 32-bit Default Maximum UBM Size	Domino 8 64-bit Default Maximum UBM Size	Domino 8.5 64-bit Default Maximum UBM Size
Windows32, Linux, AIX, Solaris, i5/OS	512 MB	1024 MB	1024 MB
zOS	400 MB	n/a	n/a
zLinux	400 MB	n/a	1024 MB

In essence, this new Domino 8 behavior obsoletes the need to use any additional INI parameters to scale the UBM size. This includes use of the INI parameters listed below:

• PercentAvailSysResources
• ConstrainedSHM, ConstrainedSHMSizeMB
• MEMAddressableMem, MEMAddressableMemSizeMB
• MEM_EnablePreAlloc, MEM_EnableSubAlloc

For more information on these parameters as well as sizing recommendations on Domino 6 and Domino 7, refer to " Recommendations for Setting NSF_BUFFER_POOL_SIZE" (#1286171) , which discusses considerations about these parameters on systems with more than 2 GB of RAM.

Note: ConstrainedSHM can still be used to control the overall amount of shared memory usage, but is not needed in Domino 8 specifically to limit the maximum UBM size. If a change to the UBM size is deemed by IBM Support to be necessary, it is recommended to use the NSF_BUFFER_POOL_SIZE_MB parameter.