I Want A Bigger SGA But My Server Won't Let Me

Well let's say you want to increase the size of the SGA on one of your instances residing on a Linux box.  So you get an updated pfile (create pfile form spfile;), edit it, increasing the sga_max_size and sga_target parameters.  Then you update the spifle (create spfile from pfile='$ORACLE_HOME/dbs/init.ora;) and restart the instance. But the new size throws an error each time you try a startup.

If you find yourself here, a quick place to look is the kernel parameter shmall.  Many times this tends to be the limiting factor.  Find out what your shmall setting is: (cat /proc/sys/kernel/shmall).  Now sum all the values of the sga_target or sga_max_size (whichever is greater) parameter for all the instance(s) you have on this server.  Don't worry about the PGA, as it does not factor into the shmall value.  Let's say your shmall value is 4194304 and the sum of all your instances is 16 gbs.  And you attempted to increase one of your instances SGA by adding 22 more gbs (this would have given you an new overall total SGA size of 38 gbs).  This of course would account for the failure during the startup attempt as your shmall setting is too small.

Now let's see what you need to do to figure out the correct shmall setting to accommodate your desires:

First you will need to know your page size (getconf PAGE_SIZE); you issue this command and find out your pagesize is 4096.

Here is the simple math to get you there:
                 
                    ( X 1024 X 1024) / 4
                                              (16 X 1024 X 1024) / 4 = 9961472

Therefore you new shmall setting will need to be 9961472 to accommodate 38 gbs of SGA.