posted 7/21/2010 by DustinRyan - Views: [4914]
In SQL Server Analysis Services 2008, we have three storage mode options available to us: Relational Online Analytical Processing (ROLAP), Multidimensional Online Analytical Processing (MOLAP) and Hybrid Online Analytical Processing (HOLAP). There are advantages and disadvantages to each, so I figured I'd take a few minutes to give a quick overview describing the storage modes and laying out some of the pros and cons of each.
Relational Online Analytical Processing (ROLAP)
The ROLAP storage mode allows the detail data and aggregations to be stored in the relational database. If you plan on using ROLAP, you need to make sure that your database is carefully designed or you'll run into some bad performance issues.
Pros:
Cons:
Multidimensional Online Analytical Processing (MOLAP)
MOLAP is the default and thus most frequently used storage mode. With MOLAP storage, the data and aggregations are stored in a multidimensional format, compressed and optimized for performance. This is both good and bad. When a cube with MOLAP storage is processed, the data is pulled from the relational database, the aggregations are performed, and the data is stored in the AS database.
Hybrid Online Analytical Processing (HOLAP)
HOLAP is a combination of MOLAP and ROLAP. HOLAP stores the detail data in the relational database but stores the aggregations in multidimensional format. Because of this, the aggregations will need to be processed when changes are occur. With HOLAP you kind of have medium query performance: not as slow as ROLAP, but not as fast as MOLAP. If, however, you were only querying aggregated data or using a cached query, query performance would be similar to MOLAP. But when you need to get that detail data, performance is closer to ROLAP.
This is by no means an exhaustive list of the details in regards to each storage mode, but I hope this has given you a good general understanding of the differences between ROLAP, MOLAP, and HOLAP. Feel free to post any comments or questions. Thanks, guys!
Here are this and some other articles on SSAS Storage Modes http://ssas-wiki.com/w/Articles#Storage_Modes