The following illustration shows a data backup cycle, where three points in time of media failure occurrences are marked for the following recovery examples. Examples of tape labels used by CONTROL to identify the individual save actions are given in parentheses on the right.
The recovery procedures outlined in the following three examples require the existence of at least one intact log image which can be used in log mode DUAL or NORMAL to recover a defective log image (see Section Backup / Restore / Devspace).
Recovery After Media Failure A
To recover the database after the media failure A, only the first backup version of the data devspace must be restored using Backup / Restore / Data. A subsequent Restart completes the data devspace by redoing the transactions recorded in the log.
Recovery After Media Failure B
When media failure B occurs, there are several ways of recovering the serverdb. The quickest method of recovery consists of reloading the database using Backup / Restore / Data and subsequently reloading the modified pages using Backup / Restore / Updated Pages. Finally, Restore / Log must be performed to restore the backups of the log segments 7 and 8.
The correct choice of the log segments 7 and 8 after restoring the pages (1.2) corresponds to the ascending serverdb version which can be found in the protocol of the corresponding data backup. A correct selection avoids failures.
Since Restore / Log overwrites the log, the current log version, which is not contained in the log segments saved so far, must be backed up to an external backup device or a host file using Save / Log (Cold). Then the backups of the log segments can be restored using Restore / Log. In log mode DUAL or NORMAL, the backup of a log segment is implicitly copied onto both log images. Once the log segments 7 and 8 are restored, the copy of the current log that was backed up using Save / Log (Cold) must be restored using Restore / Log. Restart completes the recovery.
First restore variant:
There is a choice of previous backups of the log segments which can be used for the recovery of the serverdb.
Second restore variant:
Third restore variant:
Recovery after Media Failure C
When media failure C occurs, the serverdb can be recovered in the following way:
Only the last backup version of the data devspace needs to be restored. If this version is not readable for some reason, older data backup versions can be restored which require that the corresponding log segments are redone.
First restore variant:
Second restore variant:
The same procedure must be used if organizational reasons require an older database state to be restored. Backup / Restore / Log (UNTIL) can then be used to select the point in time of the desired database state.