As you add rows to a table, the database server allocates disk space in units called extents. Each extent is a block of physically contiguous pages from the dbspace. Even when the dbspace includes more than one chunk, each extent is allocated entirely within a single chunk, so that it remains contiguous.
Contiguity is important to performance. When the pages of data are contiguous, and when the database server reads the rows sequentially during read-ahead, light scans, or lightweight I/O operations, disk-arm motion is minimized. For more information on these operations, see Sequential Scans,, Light Scans,, and Configuration Parameters That Affect Sbspace I/O..
The mechanism of extents is a compromise between the following competing requirements:
Because table sizes are not known, the database server cannot preallocate table space. Therefore, the database server adds extents only as they are needed, but all the pages in any one extent are contiguous for better performance. In addition, when the database server creates a new extent that is adjacent to the previous one, it treats both as a single extent.
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