PCTFREE and PCTUSED Tips
Mike Ault Tips -
August 5th, 2004
The PCTFREE and PCTUSED parameters
tell Oracle when to link and unlink a block from the freelist chain.
The following discussion only applies if you are not using the new
Automatic Segment Space Management (ASS Management).
The ASS management tablespace is new
in Oracleand is implemented by adding the SEGMENT SPACE MANAGEMENT
AUTO clause to the tablespace definition. ASM tablespaces automate
freelist management and remove the ability to specify PCTFREE,
PCTUSED, FREELISTS, and FREELIST GROUPS storage parameters.
The setting for PCTFREE sets the
value for the percent of a block to reserve for updates. A block
will remain on a FREELIST until it reaches blocksize *
(1-(ptcfree/100)) full or greater. Here are the main issues with
High migrated/chained rows - If PCTFREE is to small, adequate
space may not be reserved in the block for update of variable sized
rows in the block, or, may not have enough space for a complete row
insert. In this case a block chaining will occur where the data is
migrated to a new block and a pointer will be established from the
old block to a new block.
High I/O - This will result in doubling the IO
required to retrieve this data. For new data blocks, the space
available for inserts is equal to the block size minus the sum of
the block overhead (84-107 bytes) and free space (
* blocksize). When you update existing data Oracle
uses any available space in the block. So, updates will eventually
reduce the available space in a block to less than
PCTFREE, the space
reserved for updates but not accessible to inserts. This removes the
block form the freelist on which it resides.
All professional DBA?s know that minimizing
chained rows is a fundamental job role and they recognize that row
migration (chaining) is a function of:
Blocks with total filled volume
less than BLOCKSIZE ? overhead ? (blocksize*(1-(PCTFREE/100)) are
available for inserts. When you issue an INSERT statement, Oracle
checks a free list of the table for the first available data block
and uses it if possible. If the free space in the selected block is
not large enough to accommodate the data in the INSERT
statement, and the block is at least filled to the value PCTUSED,
then Oracle will remove the block from the free list. Multiple free
lists for each segment can reduce contention for free lists when
concurrent inserts take place.
After processing a DELETE or
UPDATE statement, Oracle checks to see if the space being used in
the block is now less than (BLOCKSIZE ? overhead) * PCTUSED/100. If
it is, then the block goes to the beginning of the transaction free
list, and it is the first of the available blocks to be used in that
transaction. However, it is only when the transaction commits, that
the free space in the block becomes available for other
For tables with high levels of
updates, setting PCTFREE to a high value is suggested, for blocks
which are never updated, then set this to a low value. For example,
for a high update table a setting of 40-50 is acceptable, while for
a low or no update table (such as in a data warehouse) a setting of
5 or less is acceptable. PCTUSED must be set to less than
100-PCTFREE and should be set such that PCTFREE+PCTUSED is less than
100. If PCTFREE+PCTUSED=100 this can result in see-sawing of the
block on and off the free lists which can be a performance issue.
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