A persistent layer is a data warehouse layer where data is persisted (ie never deleted).
A persistent layer is also known as:
To support operational reporting you need to store a lot of transitional information that changes frequently and has little historical or strategic value to the business. That type of stuff can significantly overburden the design and maintenance of a dimensional data warehouse, so, an ODS is used.
Implementing an ODS can significantly improve and simplify data capture for loading the data warehouse. Because you can control the design and implementation of the ODS, it can be closely integrated with the DW, such as sharing key dimensions and delta capture for fact loads.