Building the Data Warehouse

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In other cases of query processing, separating the disk-based queries from the alternate-storage-based queries is desirable. Here, one type of query goes against disk-based storage and another type goes against alternate storage. In this case, there is no need to worry about the performance implications of a query having to fetch alternate-storage-based data.


This sort of query separation can be advantageous—particularly with regard to protecting systems resources. Usually the types of queries that operate against alternate storage end up accessing huge amounts of data. Because these long-running activities are performed in a completely separate environment, the data administrator never has to worry about query performance in the disk-based environment.


For the overflow storage environment to operate properly, several types of software become mandatory. Figure 4.5 shows these types and where they are positioned.







Figure 4.5 For overflow storage to function properly, at least two types of software are needed—a cross-media storage manager and an activity monitor.


Figure 4.5 shows that two pieces of software are needed for the overflow environment to operate properly—a cross-media storage manager and an activity monitor. The cross-media storage manager manages the traffic of data going to and from the disk storage environment to the alternate storage environment. Data moves from the disk to alternate storage when it ages or when its probability of access drops. Data from the alternate storage environment can be moved to disk storage when there is a request for the data or when it is detected that there will be multiple future requests for the data. By moving the data to and from disk storage to alternate storage, the data administrator is able to get maximum performance from the system.

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