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Oracle Data block Size

When PCTUSED is ignored
PCTUSED is a storage parameter in Oracle which specifies when a database block is empty enough for Oracle to add it to the free list. When the percentage of used space in a block is greater than the PCTUSED parameter, Oracle will not add new rows to the block. The default settings for all Oracle tables are PCTUSED=40.
  1. PCTUSED is disregarded for all objects created in locally managed tablespaces when Automated Segment Space Management (ASSM) is enabled.
  2. PCTUSED is ignored for indexes: Oracle must manage the freelist re-link process for index tree management because the index node boundaries are the same as the index data block size.
Hence, PCTUSED is not used for index segments.

We allocate a tablespace called GENERAL with 8k blocksizes.

We allocate a table called TOM that has one extent of 16K and PCTFREE of 50. Three data blocks are now reserved for the TOM table.

We now create a JERRY table with one 16K extent and a PCTFREE of 10. Three data blocks are allocated for the JERRY table.

Note that TOM table and JERRY table have had varying amounts of free space reserved.