Adding Currency Conversion - Expert Cube Development with SSAS Multidimensional Models - page 200

You cannot enable Writeback on a measure group if it contains
a measure whose AggregateFunction property is set to
anything other than Sum .

When you click on OK , Analysis Services will create a new Writeback partition in
your measure group and also a new fact table in your relational data source. The fact
table does not hold the values that users write back to the cube; instead it holds delta
values so that the sum of the original values in the measure group plus the deltas
adds up to the last value written back to a cell.

Writeback can lead to poor query performance on larger measure groups,
although in the case of an exchange rate measure group this is not likely.
There are two pain points:

• When a user commits an updated cell value back to the cube, this value must

be allocated down to the granularity of the Writeback fact table. If the cell
you're writing back to is at a much higher granularity than the fact table, this
can take a very long time.