3.1. Identifying Segments, Extents, and Data Blocks

Starting with the highest level of Oracle disk-space management are tablespaces. Drilling down, you find segments that can reside in only one tablespace. Each segment is constructed from one or more extents. Each of these extents can reside in only one datafile. Thus, for a segment to straddle multiple datafiles, it must be constructed from multiple extents, which are located in separate datafiles. An extent is composed of a contiguous set of data blocks, which is at the lowest level of space management. A data block is a fixed number of bytes of disk space.

Figure 3.1 shows the relationship among segments, extents, and data blocks.

The data block size is a tablespace attribute. The SYSTEM and SYSAUX tablespaces have the database's standard data block size, defined at creation time by the initialization parameter db_ block_size. Other tablespaces can have different data block sizes, defined at tablespace creation time. A database can have as many as five data block sizes. The database always allocates and uses disk space in units of data blocks. The data block size is typically 8KB to 32KB and must be a multiple of the physical block size of the storage device. Larger data block sizes are more common in data warehousing environments where the larger block size can yield shallower Btree indexes and thus better performance.