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Show of a disk block. The page of data is loaded into a page frame. The page frame is a segment of memory that corresponds in size to the size of a page of data. Although the page of data in a virtual database system fills a role that is similar to the role played by a page of data in a traditional virtual memory system, there may be some differences between the two. For example, the page of data in a virtual database system may not be a fixed size. The attributes of a virtual-database page are determined by the structure of the virtual file. It is important to acknowledge that the size of the virtual file is not directly tied to the size of the database, and that it does not need to be large to accomplish its purpose. Growth of the database is accommodated by increasing the capacity of the physical file, rather than by increasing the capacity of the virtual file. This is important because the storage provided by the physical file is less expensive than storage provided by the virtual file. In order to demonstrate the feasibility, practicality, and benefits of the virtual database concept, a virtual database system was developed. The virtual-database concepts presented in this chapter will be illustrated through a description of this virtual database system. The remaining chapters of this thesis provide an overview of HELP, describe the implementation and evaluation of a prototype virtual database, suggest future work to be done, and offer some conclusions based on lessons that have been learned from the development of the virtual database system. 19 |
