DB2 Data Links Manager Quick Beginnings

Crash and recovery overview

If a disk that contains files referred to in a DATALINK column crashes, all the user files, along with the directory hierarchy of the file system, are destroyed. To recover from such a scenario, the administrator should make periodic backups of the file system containing the user data and directory hierarchy, so that it can be restored. The restored file system must preserve directory and file ownerships, and time stamps.

After restoring the file system, the directory structure must be brought up to the point-in-time of the crash by applying the directory changes that occurred after the file system backup was taken. After this step, the RECONCILE command must be run on all tables containing files on the damaged disk. The db2_recon_aid utility is provided to simplify this task.

Following a crash, there are three possible file states:

1. Files that are in linked state, and have the RECOVERY NO option set, are treated as follows:
   • If the file is not found on the file system, the DATALINK value will be set to NULL.
   • If the file is found, and it has READ and WRITE PERMISSION FS, no additional checks will be made to validate the correctness of the file.
   • If the file is found, and it has WRITE PERMISSION BLOCKED, its modification time and file size will be checked. If there is a mismatch in the values, the DATALINK value will be set to NULL.
2. Files that are in a linked state, and have the RECOVERY YES option set, will be restored from the archive server if the file modification time is less than the file modification time at link time, or if the file is not found.

   If the modification time of the version on the file system is greater, it is renamed with extension .MOD so that the more recent changes are not lost. The archived version is still retrieved, and the renamed version is reported in the exception report.

   If a renamed version of the file with .MOD extension already exists, the file will not be retrieved, the DATALINK value will be changed to a NULL value, and it will be reported in the exception report and table.

3. Files that are in the unlinked state on the file server are not restored or checked for correctness.