[ADMIN]Princeple of Online backup

To perform online backup we should put the tablespace in begin backup mode, copy the datafiles and then put the tablespace to end backup.
Since the size of a db block is usually of 8k bytes while the size of a os block is usually of 512 bytes, you can't guarantee that all the os blocks within a db block are at the same SCN during copying. The result is called a fractured block.

The 'alter tablespace ... begin backup' has two effects:

1. The first time a block is changed in a datafile that is in hot backup mode, the entire block is written to the redo log files. So extra redo may be generated during online backup.

2. The command prevent the SCN in datafile header from changing, so that logs are applied in recovery from backup start time.

Reference: Note 469950.1 What Happens When A Tablespace/Database Is Kept In Begin Backup Mode

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