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FREELISTS and FREELIST GROUPS. SCOPE & APPLICATION(1)
PURPOSE ======= The purpose of this article is to provide an understanding of FREELISTS and FREELIST GROUPS. SCOPE & APPLICATION =================== This article is intended to be used by DBAs to help understand how FREELISTS and FREELIST GROUP work. FREELISTS and FREELIST GROUPS ============================= The following bulletin is comprised of several parts: 1. IntrodUCtion to Oracle data block organization 2. Free List Overview 3. Free List Types 4. Considerations for Optimizing Space Management 5. Algorithms 6. FreeLists in Oracle 1. Introduction: ============= Before going into the details of freelist management one must know how the Oracle data blocks are organized. Every object in an Oracle database which occupies space is associated with a segment. A segment is a set of extents that contains all the data for a specific logical storage structure within a tablespace. There are 4 segment types: o permanent data segment (table, cluster) o index segment o rollback segment o temporary data segment Each segment is made up of smaller storage units called extents. An extent is a storage unit composed of contiguous blocks. Each segment is initially created in the database with at least one extent (controlled by parameter MINEXTENTS). Though empty, the space is reserved. When the initial space is full, a new extent is dynamically allocated. The new extent size is determined by the parameters NEXT and PCTINCREASE. The dynamic allocations stop when the number of extents reaches the value of parameter MAXEXTENTS. The first block of the first extent of each segment is reserved by Oracle to store system information and it is called the segment header. The Segment Header contains : o The Extents Table o The Free Lists descriptors o The High Water Mark (HWM) The HWM marks a limit in the segment where each block below is "in use", and each block above has never been used. The HWM always increases in size; It is reset to "zero" (position to the start of the segment) when a TRUNCATE is issued.
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