SQL Server Analysis Services (SSAS) Storage Modes

SQL Server Analysis Services (SSAS) Storage Modes


There are three standard storage modes (MOLAP, ROLAP and HOLAP) in OLAP applications.

 

Multidimensional OLAP (MOLAP)

MOLAP is the term used to refer to multidimensional online analytical processing. It extends traditional  OLAP processing through indexing directly into a multidimensional database. In MOLAP the underlying data is cached to the OLAP server, and the aggregations are precalculated and stored in the OLAP server as well. This approach optimizes response time for queries, but because of the precalculated aggregations, it does require a lot of storage space.


Relational OLAP (ROLAP)

ROLAP keeps the underlying data in the relational data system. In addition, the aggregations are calculated and stored in the relational data system. The benefit of ROLAP is that because it is linked directly to the underlying source data, there is no latency between changes in the source data and the analytic results. Some OLAP systems may take advantage of server caching to speed up response times, but in general the disadvantage of ROLAP aggregations is that because you're not leveraging the OLAP engine for recalculation and aggregation of results, analysis is much slower.

 

Hybrid OLAP (HOLAP)

HOLAP attempts to combine the best parts of ROLAP and MOLAP in order to provide the fastest performance over the largest data set possible. Aggregations are stored in the OLAP storage, but the source data is kept in the relational data store. Queries that depend on the preaggregated data will be as responsive as MOLAP cubes, while queries that require reading the source data (aggregations that haven't been precalculated, or drilled down to the source data) will be slower, akin to the response times of ROLAP.