Hi,
MOLAP and ROLAP are different ways of physically storing the Analysis Services database cubes.
The vast majority of Cubes I've been involved with use the MOLAP (Multidimensional Online Analytical Processing) storage mode.
This is due to fast query performance which is down to optimized storage, multidimensional indexing and caching. Most Cubes are refreshed over night outside office hours so the longer time it takes to refresh data in MOLAP cube is generally not a deciding factor. Analysis services cubes are generally excellent at handling large numbers of dimensions whilst minimising data redundancy provided that there is; -
1) Effective use of hierarchies
2) Effective use of attribute relationships.
3) Optimised use of the Dimension Usage Tab by ideally basing the cube on the simplest possible relationships (star schema within the data warehouse) and joining Dimensions to Measure Groups using the simplest data types (surrogate keys)
ROLAP (Relational Online Analytical Processing) is considered to be more scalable in handling large data volumes, especially models with dimensions with very high cardinality. Also the amount of time it takes to refresh the data within a ROLAP storage mode is shorter than ROLAP.
On balance I've successfully delivered multi-billion record fact tables with fast query processing using MOLAP. Where the physical storage of the MOLAP cube was significantly less than the data warehouse.
Just for completeness HOLAP (Hybrid Online Analytical Processing) as the name suggests in a hybrid between MOLAP and ROLAP. Where HOLAP stores aggregations in MOLAP for fast query performance, and detailed data in ROLAP to optimize time of cube processing.
I hope this helps,
Kind Regards,
Kieran.
Kieran Patrick Wood http://www.innovativebusinessintelligence.com http://uk.linkedin.com/in/kieranpatrickwood http://kieranwood.wordpress.com/
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