Therefore, when the existing extents of a segment are full, Oracle allocates another extent for that segment. Oracle allocates space for segments in extents. For example, each table's data is stored in its own data segment, while each index's data is stored in its own index segment. A segment is a set of extents that have been allocated for a specific type of data structure, and that all are stored in the same tablespace. Segments The level of logical database storage above an extent is called a segment. An extent is a specific number of contiguous data blocks that is allocated for storing a specific type of information.
Therefore, you should set the Oracle block size to a multiple of the operating system block size to avoid unnecessary I/O.Įxtents The next level of logical database space is called an extent. Oracle requests data in multiples of Oracle blocks, not operating system blocks. Each operating system has what is called a block size. In contrast, all data at the physical, operating system level is stored in bytes. Oracle data blocks are the smallest units of storage that Oracle can use or allocate.
This data block size should be a multiple of the operating system's block size within the maximum limit. You set the data block size for every Oracle database when you create the database. One data block corresponds to a specific number of bytes of physical database space on disk. The Relationship Among Segments, Extents, and Data Blocksĭata Blocks At the finest level of granularity, Oracle stores data in data blocks (also called logical blocks, Oracle blocks, or pages). The following illustration shows the relationships between these data structures:įigure 3 - 1. The units of logical database allocation are data blocks, extents, and segments. Oracle allocates database space for all data in a database. The Relationships Between Data Blocks, Extents, and Segments If you are using Trusted Oracle, see the Trusted Oracle7 Server Administrator's Guide for more information about storage in that environment.
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Data Blocks, Extents, and Segments Oracle7 Server Concepts Manual
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