Managing extents

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 about 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:
  • Most dbspaces are shared among several tables.
  • The size of some tables is not known in advance.
  • Tables can grow at different times and different rates.
  • All the pages of a table should be adjacent for best performance.

If you have a table that needs more extents and the database server runs out of space on the partition header page, the database server automatically allocates extended secondary partition header pages to accommodate new extent entries. The database server can allocate up to 32767 extents for any partition, unless the size of a table dictates a limit to the number of extents.

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 an extent that is next to the previous one, it treats both as a single extent.

A frequently updated table can become fragmented over time which degrades the performance every time the table is accessed by the server. Defragmenting a table brings data rows closer together and avoids partition header page overflow problems.


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