PCTFREE and PCTUSED Tips

 

Mike Ault Tips - August 5th, 2004

 

PCTFREE and PCTUSED

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 Oracle9i and 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 incorrect settings:

• 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 (PCTFREE/100 * 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 knows that minimizing chained rows is a fundamental job role and they recognize that row migration (chaining) is a function of:

• blocksize

• PCFREE

• load time average row length

• Expected row expansion (bytes per row)

 

The un-link process

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.

The Re-link Process

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 transactions.

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.