A snapshot freezes the current status of the data area. After you have made a snapshot, you can continue to work normally with the data in the database instance and make changes to the data without influencing the snapshot.
At a later time, you can do the following:
? You return to the status of the data area at the time of the snapshot, thereby rejecting all changes made since the snapshot. The snapshot contains all necessary information; you do not need a complete data backup for this.
A snapshot is not a data backup. To be able to restore your data after damages in the data area, you have to perform regular data and log backups.
? You delete the snapshot, thereby transferring all changes that were made since the snapshot to the data area.
There is only ever one snapshot. Creating a snapshot overwrites any older snapshot that may exist.
To manage snapshots, use the Database Manager GUI. See Database Manager GUI, Freezing the Data Area (Snapshots).
To create and manage the snapshot, the database system uses a shadow memory concept: a snapshot saves the current converter.
The converter maps the logical numbers of the pages containing the data to the physical pages (block addresses) in the data volumes. The converter is structured as a tree: from the starting point, links point to all logical pages in the data area, and from there to the block addresses in the data volumes.
When you create a snapshot, the database system saves a link to the current starting point in the converter in the restart information of the database instance and flags all the physical pages in the data area whose addresses are in the converter. The database system cannot overwrite these pages as long as the snapshot exists.
Data changes that you make after the snapshot are written by the database system to other physical pages in the data area. If you delete a table, the database system does not physically delete this table, but only flags it as deleted (you can no longer access this table with SQL statements). In this way, a new version of the converter is gradually created, which with time differs increasingly from the version of the converter that was saved in the snapshot.
If you reject all changes after the snapshot, then the database system deletes the starting point in the new version of the converter and replaces it with the starting point that is saved in the snapshot. All pages that changed after the snapshot are released for overwriting again.
If you delete the snapshot, the database system releases the flagged pages in the data area for overwriting.
Setting up a training computer:
1. The database administrator imports a complete data backup from the original database instance onto the training computer.
2. The database administrator makes a snapshot in the ADMIN operational state and then transfers the database instance to the ONLINE operational state.
3. The database instance is now available for training operation. The participants in the training course can change any data and start and stop the database instance.
4. After the end of the training course and before the start of the next training session, the database administrator transfers the database instance to the ADMIN operational state and discards all the changes since the snapshot.