Solaris 10, Oracle 10g, and Shared Memory

Question

Hello everyone,
We've been working on migrating to Solaris 10 on all of our database servers (I'm a UNIX admin, not a DBA, so please be gentle) and we've encountered an odd issue.
Server A:
Sun V890
(8) 1.5Ghz CPUs
32GB of RAM
Server A was installed with Solaris 10 and the Oracle data and application files were moved from the old server (the storage hardware was moved between servers). Everything is running perfectly, and we're using the resource manager to control the memory settings (not /etc/system)
The DBAs then increase the SGA of one of the DBs on the system from 1.5GB to 5GB and it fails to start (ORA-27102). According to the information I have, the maximum shared memory on this system should be 1/4 of RAM (8 GB, actually works out to 7.84 GB according to prctl). I verified the other shared memory/semaphore settings are where they should be, but the DB would not start with a 5 GB SGA. I then decided to just throw a larger max shared memory segment at it, so I used the projmod to increase project.max-shm-memory to 16GB for the project Oracle runs under. The DB now starts just fine. I cut it back down to 10GB for project.max-shm-memory and the DB starts ok. I ran out of downtime window, so I couldn't continue refining the settings.
Running 'ipcs -b' and totalling up the individual segments showed we were using around 5GB on the test DB (assuming my addition is correct).
So, the question:
Is there a way to correlate the SGA of the DB(s) into what I need the project.max-shm-memory to? I would think 7.84GB would be enough to handle a DB with 5GB SGA, but it doesn't appear to be. We have some 'important' servers getting upgraded soon and I'd like to be able to refine these numbers / settings before I get to them.
Thanks for your time,
Steven

Answer 1

To me, setting a massive shared memory segment just seems to be inefficient. I understand that Oracle is only going to take up as much memory (in general) as the SGA. And I've been searching for any record of really large shared memory segments causing issues but haven't found much (I'm going to contact Sun to get their comments).
The issue I am having is that it doesn't make sense that the DB with a 5GB SGA is unable to startup when there is an 8GB max shared memory segment, but a 10GB (and above) seems to work. Does it really need double the size of the SGA when starting up, but 'ipcs' shows it's only using the SGA amount of shared memory? I have plans to cut it down to 4GB and test again, as that is Oracle's recommendation. I also plan to run the DB startup through truss to get a better handle on what it's trying to do. And, if it comes down to it, I'll just set a really big max shared memory segment, I just don't want it to come back and cause an issue down the road.
The current guidance on Metalink still seems to be suggesting a 4GB shared memory segment (I did not get a chance to test this yet with the DB we're having issues with).
I can't comment on how the DBA specifically increased the SGA as I don't know what method they use.