sql server data warehouse: I know that I define what makes up a cube. I know that I point Analysis
Services to a data source. But when I create a cube, is it actually making a
copy of that data source defined by my dimensions and measures? Or is it
just creating meta data with my definitions?

TIA, ChrisR

a Real time ROLAP storage provide "only" a metadata around your data and
execute SQL query against the source (slow access)
a MOLAP storage get an entire copy of the data + metadata. (fast access)

in ROLAP mode you can create aggregation in the database himself, so you'll
found new tables created in your database which contains aggregated data.

MOLAP mode is recommanded by default for optimal performance.
but there is many other options regarding your needs.

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