Re: Sizing SGA

Hi Niall,

the number of incances might affect the Global Resource Directory - but I did not find any formula how to calculate it's size. Beside that I agree with Franck: shared pool will be equal for every instance, as every tatement will hit all the instances over time. Buffer cache should not decrease as well, unleass the load is really sahrded between instances. It might happen every intance has one (or more) copies of the same (busy) block in its buffer cache.

regards,
 Martin

2016-10-06 20:39 GMT+02:00 Franck Pachot <franck_at_pachot.net>:

> Hi Niall,
> IMO if the service is load balanced, the size of SGA do not depend on the
> number of nodes.
> Only when service distribution read very different data (buffer cache) or
> run different code (shared pool) you may need to increase the SGA when
> removing one node.
> I've no confidence at all in the advisors. Use them only when people wants
> me to give a number, it's still better than a random function...
> Regards,
> Franck.
>
> On Thu, Oct 6, 2016 at 3:52 PM Niall Litchfield <
> niall.litchfield_at_gmail.com> wrote:
>
>> A colleague asked me a question yesterday to which I don't really have a
>> good answer; so I thought I'd crowdsource it.
>>
>> Given a RAC database of N instances each with an SGA of M gb in size.
>> When changing the instance count N how, if at all, do you modify the value
>> of M? What metrics do you look at, and what is the rationale behind that.
>>
>> I'm aware that "let it run for a while and use the memory advisors" is an
>> approach - I can't say I have a lot of confidence in the memory advisors
>> from past experience.
>>
>> --
>> Niall Litchfield
>> Oracle DBA
>> http://www.orawin.info
>>
>

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Martin Berger           martin.a.berger_at_gmail.com
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