Oracle Datafile backup - part1

KEY DATA STRUCTURES FOR BACKUP AND RECOVERY
Before users begin to think seriously about backup and recovery strategy, the physical data structures relevant for backup and recovery operations must be identified. This section discusses the following physical data structures:

Datafiles
Control Files
Online Redo Log Files
Archived Redo Log Files
Automatic Managed Undo

DATAFILES
Every Oracle database has one or more physical datafiles that belong to logical structures called tablespaces. The datafile is divided into smaller units called data blocks. The data of logical database structures, such as tables and indexes, is physically located in the blocks of the datafiles allocated for a database. Datafiles hold the following characteristics:

• User-defined characteristics allow datafiles to automatically extend when the database runs out of space.
• One or more physical datafiles form a logical database storage unit called a tablespace.

The first block of every datafile is the header. The header includes important information such as file size, block size, tablespace, and creation timestamp. Whenever the database is opened, Oracle checks to see that the datafile header information matches the information stored in the control file. If it does not, then recovery is necessary. Oracle reads the data in a datafile during normal operation and stores it in the buffer cache. For example, assume that a user wants to access some data in a table. If the requested information is not already in the buffer cache, Oracle reads it from the appropriate datafiles and stores it in memory.