Reclaiming Oracle disk space Oracle Database Tips by Donald Burleson

Question: My tablespace is fragmented and I want to reduce the size of my dbf files to free up disk.  How do I reclaim disk space in Oracle?

Answer: In a nutshell, the best way to reclaim disk space and ensure tightly-packed rows, is to export (or copy) the tables, adjusting the storage parameters to tightly pack them onto the data blocks and to ensure that subsequent update activity does not re-fragment the data.  

I've been reclaiming disk space since the 1980's when disk was $200,000 per meg!

Oracle is designed for high performance, and Oracle does not clean up as rows are added and deleted, thus leaving a honeycomb of free space within the physical data blocks. 

As such, there are lots of opportunities for using scheduled downtime to reclaim free disk space, and reclaiming disk space is a part of the job of an Oracle DBA.  

There are several considerations on reclaiming disk space in Oracle:

• Rows per data block:  You must adjust the PCTFREE and PCTUSED freelist thresholds to tightly pack rows onto the data block, or  use ASSM.

• Free blocks within tables:  Oracle may have significant free space on the freelists, mostly empty blocks waiting to be used by a table or index.

Oracle maps the physical files (customer.dbf) to their logical "tablespace" construct and Oracle places objects (tables & indexes) into the tablespace.

Within the tablespace, objects are scattered throughout the tablespace and corresponding datafiles.  There are several types of fragmentation, most common the "honeycomb" fragmentation and the "bubble" fragmentation, where it is more difficult to reclaim wasted disk space.  Note that you can remove bubble fragmentation by using locally-managed tablespaces, and see these notes on Oracle data file fragmentation.

Tables, indexes and tablespaces will naturally fragment as a function of update activity and Oracle has many methods for reclaiming disk space and a segment advisor which will recommend when tables and indexes will benefit from a reorganization to free up disk space.

Oracle has several tools to help reclaim disk space:

• alter database datafile xxx.dbf resize yym; - This will remove space that the physical "end" if the datafile, and the command will not work if any segments extend beyond your resize boundary.

• alter tablespace xxx coalesce - This command will reclaim space from honeycomb fragmentation  

Oracle leaves the high-water mark alone after rows are deleted, and you can reclaim space at the table level with these techniques, all of which lower the high water mark for the table, thereby freeing-up the space:

• export-import - For a complete reorganization and space reclamation, export/import allows you to restructure your files and reclaim lost space.

• dbms_redefinition - This procedure will reorganize a table while it remains online for updates.

• alter table xxx shrink - If you were 10g and beyond you could use "alter table xxx shrink space compact;" syntax.

• You can look at the coalesce table syntax.  Unlike the "deallocate unused space" syntax which removes space above the high-water mark, "coalesce" puts together discontiguous fragmented extents.  There are two type of space fragmentation in Oracle. 

First is the honeycomb fragmentation, when the free extents are side by side, and the "Swiss Cheese" fragmentation, when the extents are separated by live segments.
 
alter table xxx coalesce;

You can also deallocate unused space.  Oracle notes that the "deallocate unused space" clause is used to to explicitly deallocate unused space at "the end" of a segment and makes that space available for other segments within the tablespace. 
 
alter table xxx deallocate unused space;
alter index xxx deallocate unused space;
 
Internally, Oracle deallocates unused space beginning from the end of the objects (allocated space) and moving downwards toward the beginning of the object, continuing down until it reaches the high water mark (HWM).  For indexes, "deallocate unused space" coalesces all leaf blocks within same branch of b-tree, and quickly frees up index leaf blocks for use.

Regardless of  your approach for reclaiming disk space, you will need to run complex dictionary scripts to detect the areas of fragmentation, and these scripts can also be used to reclaim the space wastage.