DB_nk_CACHE_SIZE in oracle 10g r2

Question

Good Morning everyone
From official document ... Memory Architecture
1) How the logic applied here ? i mean 32M  how it came ?
Memory consumed by manually sized components reduces the amount of memory available for automatic adjustment.
SGA_TARGET = 256M
DB_8K_CACHE_SIZE = 32M 
The instance has only 224 MB (256 - 32) and
remaining to be distributed among the automatically sized components.
Thanks in advance. !

Answer 1

SQL> select component,current_size, granule_size from V$SGA_DYNAMIC_COMPONENTS;
COMPONENT                 CURRENT_SIZE GRANULE_SIZE
shared pool                  411041792      4194304
large pool                     4194304      4194304
java pool                      4194304      4194304
streams pool                   4194304      4194304
DEFAULT buffer cache         230686720      4194304
KEEP buffer cache                    0      4194304
RECYCLE buffer cache                 0      4194304
DEFAULT 2K buffer cache              0      4194304
DEFAULT 4K buffer cache              0      4194304
DEFAULT 8K buffer cache              0      4194304
DEFAULT 16K buffer cache      33554432      4194304
COMPONENT                 CURRENT_SIZE GRANULE_SIZE
DEFAULT 32K buffer cache             0      4194304
Shared IO Pool                       0      4194304
ASM Buffer Cache                     0      4194304
14 rows selected.
SQL> alter system set db_16k_cache_size=28M;
System altered.
SQL> select component,current_size, granule_size from V$SGA_DYNAMIC_COMPONENTS where component='DEFAULT 16K buffer cache';
COMPONENT                 CURRENT_SIZE GRANULE_SIZE
DEFAULT 16K buffer cache      29360128      4194304   <-- Allocated 28M (Multiple of 4)
SQL> select component,current_size, granule_size from V$SGA_DYNAMIC_COMPONENTS where component='DEFAULT 16K buffer cache';
COMPONENT                 CURRENT_SIZE GRANULE_SIZE
DEFAULT 16K buffer cache      29360128      4194304   <-- Allocated 32M (Multiple of 4)
SQL> alter system set db_16k_cache_size=30M;
System altered.
SQL> select component,current_size, granule_size from V$SGA_DYNAMIC_COMPONENTS where component='DEFAULT 16K buffer cache';
COMPONENT                 CURRENT_SIZE GRANULE_SIZE
DEFAULT 16K buffer cache      33554432      4194304   <-- Allocated 32M (Multiple of 4)
SQL> alter system set db_16k_cache_size=31M;
System altered.
SQL> select component,current_size, granule_size from V$SGA_DYNAMIC_COMPONENTS where component='DEFAULT 16K buffer cache';
COMPONENT                 CURRENT_SIZE GRANULE_SIZE
DEFAULT 16K buffer cache      33554432      4194304  <-- Allocated 32M (Multiple of 4)
As you can see the db_16k_cache_size is always allocated in chunks of granule size (4M). Despite setting the size to 30M,31M,32M it will still allocate 32M.
Hope this is clear.
Regards,
Suntrupth