DBA Tips Archive for Oracle

  


Segment Space Management in Locally Managed Tablespaces - (Oracle 9i)

by Jeff Hunter, Sr. Database Administrator


Contents

  1. Overview
  2. Segment Space Management Options
  3. Syntax
  4. Limitations



Overview

Beginning with Oracle8i, DBAs have the opportunity of creating locally managed tablespaces which use bitmaps to manage extents. Prior to Oracle8i, DBAs only had the option of creating dictionary-managed tablespaces. Dictionary-managed tablespaces use and update the data dictionary whenever an extent is allocated, or freed for reuse, causing excessive internal locking.

New with Oracle9i, DBAs can now take it one step further by applying the same technique in determining how free and used space within a segment is to be managed within locally managed tablespaces. This article provides a brief overview of automatic segment-space management as well as some of its limitations.



Segment Space Management Options

In Oracle9i, DBAs have two choices for segment-space management; manual (the default) and auto.

Prior to Oracle9i, free lists have been the only option for managing free space within a segment. Using bitmaps, however, provide a simpler and more efficient way of managing segment free space. They not only provide better space utilization, but eliminate any need to specify and the tune PCTUSED, PCTFREE, FREELISTS and FREELIST GROUPS parameters. If any of these parameters are specified during segment creation while using the automatic segment space management option, they are simply ignored.



Syntax

  CREATE TABLESPACE users
  DATAFILE '/u10/app/oradata/ORA901/users01.dbf' SIZE 10M REUSE
  EXTENT MANAGEMENT LOCAL UNIFORM SIZE 512K
  SEGMENT SPACE MANAGEMENT AUTO
  PERMANENT
  ONLINE;
  



Limitations



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Last modified on
Friday, 20-Jan-2006 15:48:22 EST
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