Release 10.1A: OpenEdge Getting Started:
Database Essentials

Storage areas

A storage area is a set of physical disk files. A storage area is the largest physical unit of a database. With storage areas, you have physical control over the location of database objects: you can place each database object in its own storage area, place many database objects in a single storage area, or place objects of different types in the same storage area. Though you can extend a table or index across multiple extents, you cannot split them across storage areas.

Certain storage areas have restrictions on the types of extents they support. The transaction log storage area, used for two-phase commit, uses only fixed-length extents but can use more than one. The other storage areas can use many extents but only one variable-length extent, which must be the last extent.

Storage areas are identified by their names. The number and types of storage areas used varies from database to database. However, all OpenEdge databases must contain a control area, a schema area, and a primary recovery area.

The database storage areas are:

Control area
Schema area
Primary recovery area
Application data area
After-image area
Transaction log area

Control area

The control area contains only one variable-length extent: the database structure extent, which is a binary file with a .db extension. The .db file contains the _area table and the _area-extent table, which list the name of every area in the database, as well as the location and size of each extent.

Schema area

The schema area can contain as many fixed-length extents as needed; however, every schema area should have a variable-length extent as its last extent. The schema area stores all database system and user information, and any objects not assigned to another area. If you choose not to create any optional application data areas, the schema area contains all of the database’s objects and sequences.

Primary recovery area

The primary recovery area can contain as many fixed-length extents as needed, as long as the last extent is of variable length. The primary recovery area is also called the before-image area because the .bn files of its extents record data changes. In the event of a database crash, the server will use the contents of the .bn file to perform crash recovery during the next startup of the database. Crash recovery is the process of backing out incomplete transactions.

Application data area

The application data storage area contains all application-related database objects. Defining more than one application data area allows you to improve database performance by storing different objects on different disks. Each application data area contains one or more extents with a .dn extension.

After-image area

The optional after-image area contains as many fixed-length or variable-length extents as needed. After-image extents are used to apply changes made to a database since the last backup. Enable after-imaging for your database when the risk of data loss due to system failure is unacceptable.

Transaction log area

The transaction log area is required if two-phase commit is used. This area contains one or more fixed-length extents with the .tn filename extension; variable-length extents are not allowed.