Re: SWAP Space Confusion Note 1354525.1

From: Oleksandr Denysenko <odenysenko_at_gmail.com>
Date: Tue, 29 Jan 2013 12:45:44 +0200
Message-ID: <5107A858.3000906_at_gmail.com>



Chris,
first of all - move to HugePages, because of much smaller PageTables and to avoid performance penalty by addressing such large memory

second - don't allocate 80% of Linux memory to Oracle, because the rest needed not only for 'system memory', but for PageTables/Cache/processes(including Oracle) private structures like call stack, local variables e.t.c

additionally - you don't have full control over PGA size - it's only recommended value, so oracle may overuse it if more memory needed. PS:
Recently I have seen situation where 40% of 96GB RAM was dedicated to Oracle instance, and just after 15 minutes of running the application(with about of 1500 connected users) HUGE SWAPPING was started - the reason was bad SQL plans with high PGA demand

-- 
Best Regards,

     Oleksandr Denysenko

28.01.2013 21:27, Christopher.Taylor2_at_parallon.net пишет:

>
> Well, you never really want any part of your Oracle instance memory sitting in swap.That's bad
> [from a performance perspective].
>
> Typically you assign about 80% of Linux memory to Oracle if it's a dedicated Oracle
> server.(leaving 20% for system memory)
>
> **However** I did just think about needing additional space for the page tables since we're using
> 4K pages right now.
>
> I'll have to take that into account and deduct it from the 128 Ц that's about 10GB off the top
> right there just in page tables (to be on the safe side)
>
> Chris
>
> *From:*Oleksandr Denysenko [mailto:odenysenko_at_gmail.com]
> *Sent:* Monday, January 28, 2013 1:25 PM
> *To:* Taylor Christopher - Nashville
> *Cc:* oracle-l_at_freelists.org
> *Subject:* RE: SWAP Space Confusion Note 1354525.1
>
> Chris,
>
> Don't allocate 120GB of 128 GB of available memory to oracle instance or you are just asking for
> troubles...
>
> PS:
> I have bad experience of allocating 8GB to SWAP from 32GB of available memory on HP-UX 11.23,
> but I think that on Linux you may use 16GB if you are not overallocating you memory
>
> PS:
> Just a question - are disks for swap more expensive than RAM ? :-)
>
> Oleksandr Denysenko
>
> 28.01.2013 20:51 пользователь <Christopher.Taylor2_at_parallon.net
> <mailto:Christopher.Taylor2_at_parallon.net>> написал:
>
> So, going back to this...
> (env 10.2.0.4 RAC on 3 node RHEL 5.6 64-bit)
>
> On a system with 128GB RAM, 100GB SGA, 20 GB PGA, 500 connected sessions (20-30 active), how
> confident can we be that 16GB of SWAP is sufficient?
> (side note: RHEL is running at mostly default settings such as vm.swappiness = 60)
>
> I've seen notes all over the place on this and I'm wary setting the swap space too low but also
> don't want to set aside a large swap area just to have it sitting there doing mostly nothing.
>
> Chris
>
>
>
>
> Ref: 10gR2 SWAP Space Requirement For Systems Having Huge RAM [ID 1354525.1]
>
> Per the above doc, the "GOAL" section says:
> "For Systems having Huge Memory (RAM) is it really required to have SWAP Space as 0.75 X RAM"
>
> Per the Solution it says:
> Solution
> As per the Documentation below is the swap space requirement for 10gR2:
> RAM Swap Space
> === =========Up to 512 MB 2 times the size of RAM
> Between 1024 MB and 2048 MB 1.5 times the size of RAM
> Between 2049 MB and 8192 MB Equal to the size of RAM
> More than 8192 MB 0.75 times the size of RAM
>
> As part of Unpublished Documentation Bug below are the new values for SWAP Space requirements
> approved by Oracle Development Team and also updated in 11.1 and 11.2 Install guides as below:
> RAM Swap Space
> === =========Between 1 GB and 2 GB 1.5 times the size of RAM
> Between 2 GB and 16 GB Equal to the size of RAM
> More than 16 GB 16 GB
>
> All the other pre-requisites mentioned for Oracle Database Software in the installation guide
> published by Oracle Development Team need to be completed as appropriate.
>
> You may proceed with the swap space requirements as mentioned in the updated guides and ignore any
> swap related warnings during the installation of 10gR2.
>
>
>
> Chris Taylor
> Oracle DBA
> Parallon IT&S
>
>
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> http://www.orawin.info
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Received on Tue Jan 29 2013 - 11:45:44 CET

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