RE: SGA bigger than 50G

From: Powell, Mark <>
Date: Thu, 6 May 2010 12:33:29 +0000
Message-ID: <>

Nuno, well said.

-----Original Message-----

From: [] On Behalf Of Nuno Souto Sent: Thursday, May 06, 2010 7:17 AM
Subject: Re: SGA bigger than 50G

Folks, if I may interject here:

Sanjay claimed he was using Linux. Giving him information on Solaris and other Unix variants means essentially nothing and only adds to confusion: Linux does VM in a totally different manner from Solaris (which release?) and most other Unixes, like for example AIX.

So please let's not confuse things: stick to Linux. No, VM in Linux is NOT the same as *nix! There are some very big differences.

My comment regarding swap size is very simple: in some Linux releases swap is reserved to the total size of process memory even if not all used by the running processes.

And when it runs out, strange things happen: the error recovery from swap space exhaustion is not defined in any standard and various flavors of Linux and even Unix react in many different manners.

Of course swap capacity has nothing to do with Oracle SGA, but it is affected by Oracle processes' runtime memory usage and that can be a direct result of the SGA size.

Hugepages are just a special case of all this. Just as an example: AIX can page out hugepages, for them to be locked it's not enough to just declare or use them. I don't know what Red Hat does now but in earlier releases the behaviour with hugepages changed with just about every patch, so it pays to check rather than assume they behave in any given manner.

As to the need for a large SGA: without knowing the exact characteristics of the given application, it is impossible to claim it is unnecessary. And the label of "DW" means absolutely nothing: I can show you DW instances that do mostly reads, while others do mostly writes and ours just reached 4TB writes, 7 TB reads per day and 45GB SGA. And please don't claim it is or it is not a DW: it is a DW and it does what it needs to do, period.

So let's ease off the "a DW is not that" comments and stick to try and help Sanjay? Quite frankly I don't give a hoot what the accepted definition in the corridors of Oracle Acedom is for a DW: if a client says it's a DW, well, it is; end of story!

My guess is that we need to know more about what Sanjay has got there before going off on what he should have. I'd look at the hugepage usage, at the swap size use and at the shmmax setting, the flavor and release level of Linux he's using, whichever is applicable in his particular case. Only as a last resort would I suggest reducing the size of the SGA based on an assumption about the needs of his application.


Nuno Souto
in sunny Sydney, Australia


-- Received on Thu May 06 2010 - 07:33:29 CDT

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