Re: sga & shared memory

On Wed, 13 May 1998 18:27:52 -0700, Richard cummings <rich_at_ns.net> wrote:
>I recently moved into a job that includes administering a unix system
>running Oracle. I am trying to learn about the system setup and
>monitoring Oracle to make sure it runs efficiently. I remember reading
>or being told that the Oracle sga should be contained in one shared

Oracle recommends it, but I'm not sure that there are any problems if Oracle isn't complaining on startup. Shared memory doesn't page out, so once you start up, you have that memory. The problems arise when you start trying to adjust the sga, then it won't start up. There is perhaps a bit of additional fragmentation near the segment splits, I would guess. If you have multiple instances started and stopped multiple times, that could make the shared memory non-contiguous, I would guess that is the source of the advice that it is much more efficient to have each SGA in one large segment.

>memory segment. Is there a problem if the sga requires several segments

I've seen quite a bit of variance between unixes as to how the shared memory is handled, so use ipcs to check that things are working as you expect. If several small segments are right next to each other and you only have one instance and oracle releases the shared memory when you shut it down, no problem.

>to hold it? Does it hurt performance? Should I change the unix shared
>memory parameter to allow a larger size, and rebuild the kernel? Thanks.

Yes. Best not to ignore recommendations.

>

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