Re: Memory Sizing Advice

From: <>
Date: Fri, 9 May 2008 08:11:12 -0700 (PDT)
Message-ID: <>

Comments embedded.
On May 9, 9:56 am, bhonaker <> wrote:
> On May 9, 10:06 am, "" <> wrote:
> > On May 9, 8:47 am, bhonaker <> wrote:
> > > > The question I have is, is there any downside to me buying, say, a 32G
> > > > box and setting the SGA size at 20G? Will I actually end up harming my
> > > > performance with an over-large SGA (assuming I have enough physical
> > > > memory to keep the box out of swap)?
> > > Since everyone is busy telling you how to tune instead of answering
> > > your question, you might have to infer that the answer is "No, there
> > > is no downside to adding memory."  That's my takeaway from no
> > > negatives pointed out anyway...
> > Then you're not reading the entire thread, as I posted that installing
> > all of the physical memory a server can accept, then allocating 80% of
> > that to the database would be wasteful, to say the least.
> To me "wasteful" does not translate to "harming performance"

In some cases you're correct, in others you may be wrong. Since you can't witness the affected system 'in action' you can't comment either way.

> > Knowing that this is a Windows operating sytem, which requires 2 gig for the
> > operating system alone, may make that 80% allocation 'impossible' thus
> > creating a scenario of constant paging/swapping to/from disk.  Of
> > course even a successful allocation of that much memory to the SGA
> > would create a paging/swapping situation as PGA components may require
> > more free memory than is available.  Which, in turn, sends performance
> > into the proverbial dumpster.
> > Even if he's lucky and no paging occurs it's highly likely his memory
> > allocations will be unused as constantly changing data causes the
> > cache to be refreshed from disk, thus killing the 'benefit' of having
> > all of those lovely data blocks in cache.  And bloating the SGA to
> > starve the O/S is ... not the wisest of moves.
> Pat suggested "a 32G box and setting the SGA size at 20G."  Are you
> saying that 12G for the OS would be starving it?

Certainly not, and if you read it that way you need to look at the context again. That comment was made with reference to the previous paragraph where, on a smaller 'box', the maximum RAM installable may be 8 gig, and 80% of that would leave less than 2 gig for the O/S, which COULD starve it for resources.

> > The negatives of this situation are known by most of those who have
> > posted to this thread.  Siimply because you can't see them in print is
> > no indication they don't exist.
> > David Fitzjarrell
> I understand that 99% of the posters here are more knowledgable than
> me.  I am not trying to be argumentative, I simply haven't seen any
> negatives listed - perhaps I need to learn to read between the lines
> better?  All I see is people saying it is wasteful and that there are
> better ways to spend time and money, but I haven't seen anyone say
> "The reason you DON'T want a 20G SGA allocated out of 32G is because
> this will happen and this will happen, you would be better off with a
> 10G SGA allocated out of 32G."

As I said before since you, nor anyone else responding to this thread outside of the O/P, can actually witness this configuration in action it would be foolish to make such a claim.

David Fitzjarrell Received on Fri May 09 2008 - 10:11:12 CDT

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