Dictionary cache is information about the databse objects stored in a data dictionary table.
Row size also reduces no of rows per block.
A free extent is a collection of continuous free blocks in tablespace. When a segment is dropped its extents are reallocated and are marked as free.
For perticular transaction Alter system set rollback segment 'name';
For database, we can set in pfile. Rollback_segment='name'
Store data in tablespaces to avoid disk contension.system tablespace-recursive callsuserdata-users objectsindex tablespace-for indexes rollback segmnets-undo tablespace or manual rollback segmentsplace application specific data in respective tablespaces.Place all these tablespaces in saperate disks.Try to implement raid-0 (striping) for better performance.