About Data Files

Data files are the operating system files that store the data within the database. The data is written to these files in an Oracle proprietary format that cannot be read by other programs. Tempfiles are a special class of data files that are associated only with temporary tablespaces.

Data files can be broken down into the following components:

• Segment

  A segment contains a specific type of database object. For example, a table is stored in a table segment, and an index is stored in an index segment. A data file can contain many segments.

• Extent

  An extent is a contiguous set of data blocks within a segment. Oracle Database allocates space for segments in units of one extent. When the existing extents of a segment are full, the database allocates another extent for that segment.

• Data block

  A data block, also called a database block, is the smallest unit of I/O to database storage. An extent consists of several contiguous data blocks. The database uses a default block size at database creation.

  After the database has been created, it is not possible to change the default block size without re-creating the database. It is possible, however, to create a tablespace with a block size different than the default block size.

Segments, extents, and data blocks are all logical structures. Only Oracle Database can determine how many data blocks are in a file. The operating system recognizes only files and operating system blocks, not the number of data blocks in an Oracle Database file. Each data block maps to one or more operating system blocks.