You can perform an online backup with the database running – that is, the users can continue to work normally. The management of database changes by the corresponding Oracle background processes is not affected.
Tablespace online backups alone are inconsistent. To make the database consistent, you need to apply redo log entries from the run-time period of the backup. If you use Oracle Recovery Manager (RMAN) for online backup, it takes care of internal block consistency during the backup.
An online backup is made using operating system tools such as cpio or dd – for example, under the control of BRBACKUP. Since these tools are not part of the database system, Oracle must be informed about the starting point of a backup. In this way, a unique starting point is defined from which the changes of all the data in a tablespace can be recovered in the event of an error. This process works as follows:
1. The starting point of the backup is set using the command ALTER TABLESPACE <tablespace name> BEGIN BACKUP. The header of the tablespace files holds information on the checkpoint and redo log files, that is, the system change number (SCN). When the next redo log file switch or checkpoint occurs (normal database operations continue), the header information remains unchanged.
2. Based on this mechanism, all the files of a tablespace are copied with uniform header information specifying when the backup was started.
3. Once the backup of the tablespace is complete, the command ALTER TABLESPACE <tablespace name> END BACKUP makes sure that the header information of the files is updated.
RMAN normally takes care of this process internally, so it is not explicitly performed.
For more information on what to do if your database crashes during an online backup, see Fixing an Online Backup Crash.
After an offline backup of the complete database, you have a backup of the database that is consistent. If you work with the database after the backup, the backup is no longer up-to-date. In this case, you have to recover the database after you restore the backup, using the redo log files.
You must close the database for an offline backup, which means that you have to stop work in the SAP System. However, the SAP System does not have to be shut down for an offline backup. If the RECONNECT mechanism is set in the SAP start profiles, the connection to the SAP System is remade after the database is restarted. This means that the information in the buffers of the SAP System is not lost, which implies better performance immediately following the database startup.