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.