Database Reference
In-Depth Information
Database
The Oracle Database includes all the files that constitute the dataset. This includes the data-
files, control files, online redo log files, and offline archive log files. When an Oracle sys-
tem is started, the database files are associated with the Oracle instance, and any operations
the user performs are recorded in the appropriate files at the operating system level. The fol-
lowing definitions will clarify the different files that constitute the Oracle Database.
Tablespaces
Oracle manages data in logical units referred to as tablespaces. A logical tablespace consists
of one or several physical files referred to as datafiles. When a new database object, such as
a table or an index, is created this occurs in an assigned tablespace. When the tablespace is
running out of space, it can be extended by adding datafiles. SAP follows specific naming
conventions for the tablespaces.
Datafiles
Datafiles are actual physical files at the operating system level with set naming conventions
to which the data is saved by database operations. Datafiles can be added to the tablespace
as long as the mount point has space allocated and is available for such growth. In real-life
production SAP systems, there will be tens and hundreds of datafiles depending upon the
database growth of a given business operation.
Control Files
A control file has the list of physical datafiles and the paths where such files are located.
When an Oracle instance is started, the system reads the control file to open the datafiles
and redo log files and make it available for general database operations. Since the control
file is such an important file, usually more than one copy exists at different locations at the
operating system level.
Redo Log Files
When data is changed in the Oracle system, it is not permanently written to the datafiles
right away. The data changes are written to the redo log files. In case there is a power fail-
ure, for example, data is applied back from the redo logs at instance start. In this way any
data loss is prevented. Modified data is not written on a synchronous manner permanently to
the datafiles immediately because of performance reasons. Instead, the data is written to the
datafiles from time to time.
