When an SAP liveCache database is started, the data cache and converter are dimensioned in accordance with the CacheMemorySize database parameter, whereas the size of the OMS heap is undefined. The OMS heap can grow dynamically until it reaches the maximum size specified in the SAP liveCache parameter OmsMaxHeapSize.
An SAP liveCache database works at an optimized level if the SAP liveCache data is accessed in the main memory only. Avoid accessing the data in the data area. For this reason, when you analyze SAP liveCache performance, first analyze the main memory areas data cache and OMS heap.
The memory area of the data cache should take up about 40 % of the main memory available to a SAP liveCache database. You may need to adjust its size to the requirements of the application. Among other things, the following data is stored in the data cache:
More information about OMD data: Glossary, OMS, and OMS Version.
Links to Undo Log entries
More information about undo log entries: Concepts of the Database System, History Management.
Just as in an OLTP database system, the SQL data is organized in database tables and, if required, indexes. The data is managed in SQL data pages in the form of B* trees. These B* trees can contain the following data: the keys of the objects, the database tables and their indexes, and the OMS versions for which swapping was performed. Usually, only about 2-3% of the total data cache is used for SQL data.
The data cache must be dimensioned so that it can hold all OMS data, Undo Log information and SQL data in the data cache. Otherwise, the data is written to the data area, which negatively affects the performance of the SAP liveCache database. This should hardly ever occur.
The data cache hit rate should be at least 99,8 %.
The data cache load should be significantly less than 100 %.
The amount of OMS data in the data cache should be significantly higher than the amount of links to Undo Log entries (a ratio of 4:1 is normal).
To monitor the data cache, you can use the Caches user menu ( DBA Cockpit/ liveCache Assistant) or the displays Caches and Memory in the liveCache Alert Monitor.
The data cache hit rate is below 99.8%, the data cache load is 100%.
To display the failed accesses to the data cache, refresh the Caches display. Check whether there are any long-running OMS versions ( Monitoring OMS Versions). If the measures described in Monitoring OMS Versions do not improve the critical situation, increase the dimensions of the data cache ( Caches ( DBA Cockpit/ liveCache Assistant)).
The amount of OMS data and Undo Log entries is about the same or there are more Undo Log entries than OMS data.
Check the OMS versions that generated the Undo Log entries. Follow the procedure described in Monitoring OMS Versions.
To monitor the OMS heap, use the OMS Heap Usage display (DBA Cockpit)or Heap Usage (liveCache Assistant), or the Memory display in the liveCache Alert Monitor.
The fill level of the OMS heap is greater than 80 %.
Perform other analyses as described in Monitoring OMS Versions.
The fill level of the OMS heap is greater than 90%.
DB procedures usually terminate if the OMS heap has almost reached its maximum value.
You can reduce the size of the data cache (general database parameter CacheMemorySize) and increase the OMS heap (general database parameter OmsMaxHeapSize) instead.
You can make more main memory available to increase the size of the OMS heap (general database parameter OmsMaxHeapSize).
Performance Analysis in CCMS: SAP liveCache Technology
Database Administration, General Database Parameters, Special Database Parameters