db_block_size

Lowest level of database storage. Should be multiple of the OS block size. Set at database creation, it can never be changed.

PCTFREE allocates percentage of block space that remains free for future updates. One the PCTFREE threshold has been achieved, no new rows are added to the block, only updates to existing rows in the block can occur.

PCTUSED specifies how much space must be freed from a block (through deletions, etc...) before new rows can be added to a block.

Online transaction heavy systems benefit from a small block size. Batch processessing system benefit from larger block sizes.