This parameter in the init.ora is the most important. This can be done only during creation time. If you have already created the Database you cannot change this value. You will have to re-create the Database with a different size.
When you start to think about larger block sizes, remember that a 32KB undo block size can be a source of wasted I/O.
This block size is used for the SYSTEM tablespace and by default in other tablespaces.
Solaris Sparc 64-bit
Oracle 12c
SQL> conn / as sysdba
Connected.
SQL> show parameter db_block_size
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_block_size integer 32768
SQL>
