Use MOLAP storage mode for dimensions when those dimensions are related to measure groups that contain semi-additive measures
This rule analyzes database dimensions to determine which ones contain semi-additive measures and meet one of the following conditions:
The dimension has ROLAP storage specified.
The dimension has proactive caching enabled, and the proactive caching settings have the OnlineMode property set to Immediate.
For best performance, you should typically use MOLAP storage for all database dimensions that have measure groups that contain semi-additive measures. If you enable proactive caching on the dimension, you should also avoid proactive caching settings that enable the dimension to enter the ROLAP mode. (That is, avoid setting the OnlineMode property to Immediate.)
For more information about how to configure proactive caching, see Proactive Caching in SQL Server Books Online.
For more information about performance issues related to proactive caching, see the section, "Near real-time data refreshes," in the SQL Server 2005 Analysis Services Performance Guide.