Journal access paths

Journaling access paths can significantly reduce recovery time by reducing the number of access paths that need to be rebuilt after an abnormal system end. It is recommended that you journal access paths because larger access paths require more time to rebuild.

When you journal database files, you record images of changes to the file in the journal. The system uses these record images to recover the file after an abnormal system end.

After an abnormal end, the system might find that access paths are not synchronized with the data in the file. If an access path does not synchronize with its data, the system rebuilds the access path to ensure that the two are synchronized and usable.

When journaling access paths, the system records images of the access path in the journal to provide known synchronization points between the access path and its data. By having that information in the journal, the system recovers both the data files and the access paths. The system then synchronizes the two. In such cases, you avoid the lengthy time to rebuild access paths.

In addition, other system recovery functions work with journaling access paths. For example, the system has a number of options to reduce recovery time from the failure and replacement of a disk unit. These options include user auxiliary storage pools and checksum protection. These options further reduce the chances that the entire system must reload because of the disk failure. However, you might still need to rebuild access paths when the system is started following replacement of the failed disk. By using access path journaling and some of the recovery options, you reduce your chances of having to reload the entire system and having to rebuild access paths.

Before journaling an access path, you must journal the physical files that are associated with the access path. In addition, you must use the same journal for the access path and its associated physical files. It is easy to start journaling access paths:

You can use the system-managed access-path protection (SMAPP) facility.
You can manage the journaling environment yourself with the Start Journal Access Path (STRJRNAP) command.
- To start journaling the access path for the specified file, use the STRJRNAP command. You can journal access paths that have a maintenance attribute of immediate (*IMMED) or delayed (*DLY).
- After you start journaling, the system protects the access path until the access path is deleted or until you run the End Journal Access Path (ENDJRNAP) command for that access path.

Access path journaling minimizes additional output operations. For example, the system will write the journal data for the changed record and the changed access path in the same output operation. However, you should seriously consider isolating your journal receivers in user auxiliary storage pools when you start journaling your access paths. Placing journal receivers in their own user auxiliary storage pool provides the best journaling performance, while helping to protect them from a disk failure.