RE: multi-tenancy
Date: Tue, 7 May 2013 13:21:33 -0500
Message-ID: <F05D8DF1FB25F44085DB74CB916678E8879E0DB715_at_NADCWPMSGCMS10.hca.corpad.net>
Oracle recommends no more than 80% of system memory for ALL SGAs (OLTP) on a server as a starting point. The remaining 20% should be used for PGA, os processes etc.
(source: Automatic PGA Memory Management [ID 223730.1)
Now, there are several caveats to this that you should be aware of.
a.) One of the primary ones is do you use dedicated server connections or mts, and you need to know the peak value of connected processes (or a least a reasonable number). It is possible to size your SGAs/PGAs within the guidelines set by Oracle and yet still send your server into a tailspin because you have hundreds/thousands of connected sessions, all using memory segments.
b.) If you have HUGE RAM on the server, and you have large SGAs setup but you are *not* using hugepages, then your pagetables (thinking of Linux here) will be huge and pagetables reside in memory and thus decrease the amount of RAM available to other processes.
For example, on one of my systems that I'm working into shape: The SGA is ~90GB, the SYSTEM RAM is 128GB, and the PAGETABLES are approximately 10GB using 4K page sizes (it's a work in progress!). Anyway, the amount of "useable" RAM is around 118 GB, not 128GB right off the top as pagetables are consuming approximately 10GB.
So, in a nutshell, there are a lot of considerations.
If I were in your shoes, I would consider that I have 3 databases, and "X" amount of RAM. I would multiply "X" by 0.80 to come up with the absolute MAXIMUM memory I can use for Oracle SGAs on this box. (If you're using Linux and have small, default pages and a lot of system RAM, then I would probably automatically deduct about 10GB off that number for pagetables)
128 GB (-10GB for pagetables if using small pages) * 0.80 = ~95GB of RAM available for 3 SGAs and go from there.
Just some thoughts.
Chris
-----Original Message-----
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Zelli, Brian
Sent: Tuesday, May 07, 2013 9:47 AM
To: oracle-l (oracle-l_at_freelists.org)
Subject: multi-tenancy
I have two instances, 10g and 11g on one server and now they want me to add another 11g instance. Is there some white paper on how to parse up the RAM on the server to even distribute to all 3 instances? Some formula? This is all development so I don't have any ideas of size and such.....neither do they.
ciao,
Brian
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Received on Tue May 07 2013 - 20:21:33 CEST