The ODS sits between the legacy applications environment and the data warehouse. The ODS is necesary when you have multiple and different kind of source systems, so that you can have data from all the sources in one place. normally ODS feeds the data warehouse
we cannot say exactly that ODS is mirror of the data warehouse. but we can say that ODS is also a data warehouse but the only difference between ODS and Data warehouse is ODS is Volatile,
normally ODS is loaded on daily basis sometimes it is updated as soon as source is updated, normally it is done on incremental basis
but Data warehouse will be updated periodically, may daily or weekly or monthly it depends on your data warehouse grain
you can check more details about the ODS in articles on
those articles give you the clear idea than my explanation, if you have any question just ask me
NOTE:
if you want to know about the concepts and details of data warehouse i dont suggest you to read any book which is specific to the particular tool, try reading some general books on data warehouse so that you can get the clear concept of it and then you can continue reading your books on particular tools.
ô¿ô Kishore