Building the Data Warehouse

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Data that has a high probability of access and a low volume of storage resides on a medium that is fast and relatively expensive. Data that has a low probability of access and is bulky resides on a medium that is cheaper and slower to access. Usually (but not always) data that is older has a lower probability of access. As a rule, the older data resides on a medium other than disk storage.


DASD and magnetic tape are the two most popular media on which to store data in a data warehouse. But they are not the only media; two others that should not be overlooked are fiche and optical disk. Fiche is good for storing


customer

Figure 2.8 The subject area may contain data on different media in the data warehouse.


detailed records that never have to be reproduced in an electronic medium again. Legal records are often stored on fiche for an indefinite period of time. Optical disk storage is especially good for data warehouse storage because it is cheap, relatively fast, and able to hold a mass of data. Another reason why optical disk is useful is that data warehouse data, once written, is seldom, if ever, updated. This last characteristic makes optical disk storage a very desirable choice for data warehouses.


Another interesting aspect of the files (shown in Figure 2.8) is that there is both a level of summary and a level of detail for the same data. Activity by month is summarized. The detail that supports activity by month is stored at the magnetic tape level of data. This is a form of a “shift in granularity,” which will be discussed later.

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