PCTFREE, PCTUSED and ASSM

From: Mladen Gogala <mladen_at_wangtrading.com>
Date: Tue, 11 Nov 2003 12:24:33 -0800
Message-ID: <F001.005D6576.20031111122433@fatcity.com>

Int the note 247752.1 named "Oracle9i Space Management Demystified" oracle says the following:

---

AUTOMATIC SEGMENT SPACE MANAGEMENT
ARCHITECTURE Oracle9i introduces a new way of managing free space within a segment using bitmaps. In the new scheme, a set of bits describes the space utilization for each block in a segment as well as whether it is formatted or not. Using these Paper # 32707 Oracle9i Database bits, for example, it is possible to represent the state of any given data block as follows: " Free space in the block is less than 25% " Free space in the block is greater than 25% and less than 50% " Free Space in the block is greater than 50% but less than 75% " Free space in the block is more than 75% " The block is FULL i.e. there is no free space in the block " The block is unformatted. It can be noted here that unlike freelists, where a block was either available for new rows or not, bitmaps provide a more granular and accurate idea of space utilization within blocks of segment. For LOBs and indexes, the bitmap just indicate " Whether the block is formatted or not and, " Whether the block is considered free or not. The Automatic Segment Space Management feature can only be used with Locally Managed Tablespaces. BITMAP BLOCKS The bitmaps are contained in a set of meta-data blocks known as bitmap blocks or BMBs. The number of BMBs in a segment depends on its size and the space consumed by the bitmap blocks is typically a very small part (less than 1%) of the total segment size for any reasonable sized segment. As shown in the table below, the space overhead of bitmap blocks decreases as the segment grows and becomes close to negligible for large segments.
Segment Size Block Size No. Of BMBs Space Used by BMBs BMB Space Overhead

25 MB        2 KB       201         400 KB             .8% 
500 MB       8 KB       251         2   MB             .4% 
100 TB       16KB       6555941     100 GB             .05% 
------------------------------------------------------------------------------------

>From that, it would follow that free lists as such are gone in ASSM tablespaces
and are replaced by bitmaps. As Richard Foote has shown, PCTFREE is not ignored, but without free lists, it doesn't make much sense. Does any of the gurus (Cary, Steve, Jonathan, Wolfgang, Pete Sharman) have any knowledge of how exactly PCTFREE is implemented in the ASSM situation? I can see and verify that it is not ignored, but I can't figure how does it work.

Mladen Gogala
Oracle DBA

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Author: Mladen Gogala
  INET: mladen_at_wangtrading.com

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Received on Tue Nov 11 2003 - 14:24:33 CST