Have you done any analysis which warrants an increase in sga size ?

Firstly check the main memmory areas to see whether they are hitting the recommended statistics.

No need to increase if your system is making optimal use of its memmory structures. Below are a few scripts to get you started.


prompt Listing Buffer Cache Hit Ratio
prompt Ideally this should be above 90%
prompt ---------------------------------


column "logical_reads" format 99,999,999,999
column "phys_reads" format 999,999,999
column "phy_writes" format 999,999,999
select a.value + b.value "logical_reads",
c.value "phys_reads",
round(100 * ((a.value+b.value)-c.value) /
(a.value+b.value))
"BUFFER HIT RATIO"
from v$sysstat a, v$sysstat b, v$sysstat c
where
a.statistic# = 38
and
b.statistic# = 39
and
c.statistic# = 40;

prompt
Prompt Listing Get Hit Ratio of Library Cache
Prompt Ideally this should be above 90%
Prompt -------------------------------------

select gethits,gets,(gethitratio*100)
from v$librarycache
where namespace = 'SQL AREA';

prompt
prompt Listing Reloads/Pins Ratio of Library Cache
Prompt Ideally this should be below 1%
prompt ---------------------------------------------

select reloads,pins,(reloads/pins * 100) AS RATIO
from v$librarycache
where namespace = 'SQL AREA';

prompt
prompt Listing Getmisses/Gets ratio of Dictionary Cache
Prompt Ideally this should be below 15%
Prompt ------------------------------------------------

select sum(getmisses),sum(gets),sum(getmisses)/sum(gets) * 100
from v$rowcache;

prompt
Prompt Listing Disk/Memmory Ratio of Sort Area Size
Prompt Ideally this should be below .10
Prompt -------------------------------------------------

SET HEADING ON
SET FEEDBACK OFF
COLUMN name FORMAT a30
COLUMN value FORMAT 99999990

SELECT name, value
FROM v$sysstat
WHERE name IN ('sorts (memory)', 'sorts (disk)');

prompt
prompt
Prompt Listing redo log space request/entries ratio
Prompt Ideally this should not be above 0.0002
prompt ---------------------------------------------

SELECT REQ.VALUE/ENTRIES.VALUE "SPACE REQUEST RATIO"
FROM V$SYSSTAT REQ,V$SYSSTAT ENTRIES
WHERE REQ.NAME='redo log space requests'
AND ENTRIES.NAME='redo entries';