Relational Databases - IBM Mainframe

The DB2 data storage has two aspects one is how the user sees it(the logical storage)second is how it is actually stored(the physical storage). This section,in a very limited way, deals with the physical organization of data. The following figure is a representation of how the physical storage is implemented.

The total collection of stored data is divided into a number of user databases and system databases. Each database is divided into a number of tables paces and index spaces. A 'space' is a dynamically extendible collection of pages, where a 'page' is a block of physical storage. The pages in a given space are all of the same size 4KB for index spaces and either 4KB or 32KB for table spaces. Each table space contains one or more stored tables. A stored table is the physical representation of a base table, it consists of a set of stored records, one for each row of the base table. A given stored table must be wholly contained within a single table space. Each index space contains exactly one index. A given index must be wholly contained in an index space. A given stored table and all its associated indexes must be wholly contained within a single database. Views are not physical objects at all. They are included in the figure for completeness and to illustrate the fact that a view can span multiple databases. Each table/index space has an associated storage group. A storage group is a collection of direct access volumes all of the same device type. When a given space needs to be extended, storage is acquired from the appropriate storage group. The spaces in a given database do not all have to have the same storage group nor do all the spaces that share a given storage group have come from the same database.


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