Building the Data Warehouse

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To make data accessible throughout the system and to place the proper data in the proper part of storage, software support of the alternate storage/near-line environment is needed. Figure 4.4 shows some of the more important components of the support infrastructure needed for the alternate storage/near-line storage environment.


Figure 4.4 shows that a data monitor is needed to determine the usage of data. The data monitor tells where to place data. The movement between disk storage and near-line storage is controlled by means of software called a crossmedia storage manager. The data in alternate storage/near-line storage can be accessed directly by means of software that has the intelligence to know where data is located in near-line storage. These three software components are the minimum required for alternate storage/near-line storage to be used effectively.


In many regards alternate storage/near-line storage acts as overflow storage for the data warehouse. Logically, the data warehouse extends over both disk storage and alternate storage/near-line storage in order to form a single image of data. Of course, physically the data may be placed on any number of volumes of data.


An important component of the data warehouse is overflow storage, where infrequently used data is held. Overflow storage has an important effect on granularity. Without this type of storage, the designer is forced to adjust the level of granularity to the capacity and budget for disk technology. With overflow storage the designer is free to create as low a level of granularity as desired.


Overflow storage can be on any number of storage media. Some of the popular media are photo optical storage, magnetic tape (sometimes called “near-line storage”), and cheap disk. The magnetic tape storage medium is not the same as the old-style mag tapes with vacuum units tended by an operator. Instead,

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