Locally-Managed Tablespaces

Prior to Oracle8i, management of free and used extents relied heavily on the data dictionary tables. Oracle8i introduces Locally-Managed Tablespaces with an improved space management implementation. This new tablespace type automatically sizes all new extents at standard sizes, improving space allocation performance and eliminating free extent fragmentation issues.

Oracle sizes new extents using either a uniform or automatic policy. If the tablespace uses the uniform policy, all extents are the same size, which is set at tablespace creation time. Since all extents are the same size, no fragmentation can occur. If the tablespace uses the automatic policy, there are only small and large extents. Small extents are automatically used for small database objects, and large extents are used for large objects. Small extents are packed together in a group the size of a large extent. By separating small and large extents, fragmentation is avoided.

The new type of tablespace is called Locally Managed because all extent information is tracked in the tablespace itself using bitmaps. Bitmaps manage space allocation very efficiently, and require no dictionary access and update to allocate an extent to a object. This minimizes access to the data dictionary, improving availability. Because of these improvements, Oracle recommends using Locally-Managed Tablespaces for all new tablespaces if fragmentation is expected to be an issue.