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.