Determining the Size of the Database › General Database Sizing Considerations › Sizing Considerations for Compressed and Variable Length Entities

Sizing Considerations for Compressed and Variable Length Entities

Internally, the DBMS treats the following types of entities as variable in length:

• Fixed-length compressed entities—Entities with a fixed length that are compressed through a compression routine; although the length of these entities is fixed from the point of view of user programs, compression makes them internally variable.
• Variable-length entities—Compressed or uncompressed entities with a length that depends on a variably occurring data element (that is, entities that contain an OCCURS DEPENDING clause).

Fragmentation

The DBMS fragments a variable-length entity occurrence when it is unable to store the entire entity on a single page. Fragmentation forces the system to perform two or more I/Os to retrieve a single variable-length entity. Fragmentation should be kept to a minimum.

Root and fragment size

In a non-SQL environment, you can specify the following information in the schema:

• Minimum root— The smallest amount of data to be stored on the entity's home page (target page)
• Minimum fragment— The smallest amount of data to be stored on any additional page

For SQL compressed entities, the minimum root and fragment are assigned automatically.

If a variable-length root or fragment exceeds 30 percent of the page size, space management problems can occur. To ensure efficient space management, you need to tailor the size of the minimum root and fragment to the optimal page size for the database area.

Page reserve

When a database area contains variable-length entities, and a general increase in the size of the entities is anticipated, you should define a page reserve in the area definition. By specifying a page reserve, you can minimize fragmentation of variable-length entities.

The page reserve is a specified number of bytes per page that can be used only for expansion of variable-length entities or internally-maintained index records. For further information on internally-maintained index records, see "Allocating Space for Indexes" later in this chapter. The space will not be used for storing new entity occurrences. In general, page reserve should always be less than 30 percent of the page size.

The page reserve is specified in the CREATE/ALTER AREA statement of the physical database definition.

Note: A page reserve does not affect the physical structure of the database. You can, therefore, vary the page reserve by using different DMCL modules, each with a different page reserve.

More Information For more information on the physical database definition, see the CA IDMS Database Administration Guide.