Release 10.1A: OpenEdge Data Management:
Database Administration

> ## After-image areas and extents

> You define AI areas in the structure description (`.st`) file. You can create a single AI area or multiple areas on a single disk or multiple disks. With multiple areas, you can perform online backups while after-imaging is enabled. Regardless of how many AI areas are defined, each AI area contains only one extent. Before defining the AI extents, consider the following:

> - The database engine fills the AI areas in the order that you define them in the structure description file. When you are defining areas, you can store more than one AI area on a disk. However, you should store all the AI areas on disks other than the one that contains the database (DB) files or primary recovery area (BI) files.

> - For both fixed-length and variable-length extents, the database engine automatically switches extents when the current extent becomes full, as long as the next extent is empty. If you define three large fixed-length extents, you can use extent 1 for a full day’s worth of transactions, and have extent 2 empty and ready to use when you need to switch over to the next extent. This also leaves extent 3 available if you perform an unusually high number of transactions and use both extents 1 and 2.

> The database engine uses AI areas sequentially, in the order defined in the structure description file. AI area filenames have a .a`n` extension, where `n` indicates the numerical order in which you defined the area. After it uses the last area, the database engine reuses the first area if it is empty. Figure 7–1 illustrates this behavior. An extent switch is the operation of switching from one AI area extent to another.

> Figure 7–1: After-image extents switching

> You must monitor the status of the extents to ensure that you do not try to reuse an unavailable file. For information on monitoring the status of your AI extents, see the "Monitoring AI file status" section.

> Like database and BI extents, there are two types of AI extents:

> - Fixed length

> - Variable length

> ##### Fixed-length extents

> Fixed-length extents are extents that are preallocated and preformatted. With fixed-length extents, you control how much disk space each extent uses, by specifying the size in the structure description file.

> ##### Variable-length extents

> Variable-length AI extents do not have a predefined length. They continue to fill until they use the entire disk, you back up the database, or you issue the RFUTIL AIMAGE NEW command. The initial length of a variable-length extent is 128K. You can define more than one variable-length AI area for a single database.

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