Re: 12c pluggable database shared SGA question

  • From: Debaditya Chatterjee <debaditya.chatterjee@xxxxxxxxx>
  • To: Glenn.Travis@xxxxxxx
  • Date: Wed, 10 Sep 2014 09:55:33 -0700

Hi Glenn,

You are correct. Environments that are similar in nature or share the same
SLA's are the best candidates for consolidation. Consolidating a full scale
ERP with a large DW in the same container may result in performance issues.

Having said that we do see some customers considering to consolidate few
very disparate applications as well.

I agree with your observation. We are currently in the process of updating
these slides so we will keep your feedback in mind.

Thanks
Deba

Debaditya Chatterjee

On Wed, Sep 10, 2014 at 9:11 AM, Glenn Travis <Glenn.Travis@xxxxxxx> wrote:

> 12c shared resources:
> "PDBs share UNDO, REDO and control files."
> "PDBs share common SGA and background processes."
>
> The selling point is that only small increments of memory are added as
> additional PDBs are added.  BUT
>
> A problem I have with the 12c 'pluggable' database, is that is shares an
> SGA - that is library cache, and buffer cache.  The diagrams used in all
> the promo (and training) materials show an ERP, CRM and DW type databases
> sharing the same memory.   This seems counter intuitive to everything we as
> DBAs have been taught for many years.  Those databases have completely
> different users and usage patterns/requirements.  I realize the PDBs are
> not sharing the buffers and statements between them, but they ARE sharing
> the memory footprint, and there is only so much memory.
>
> See slides here :
> [11g] http://goo.gl/wQ612C  vs. [12c] http://goo.gl/eshQTA
>
> Obviously an ERP is queried (and tuned) differently (single transactions,
> high volume, key values, shared sql) than a DW (multi, complex
> transactions, low number of users, long running statements, full
> table/index scans, low key value usage, and non-sharable sql).
>  Co-existence of these types of environments will not only be impossible to
> tune in one SGA, but the cache will be useless.  The DW will flush out all
> the 'good' buffers/pages used by the ERP/OLTP application users, and the
> non-sharable sql will flush out and fragment the library cache.  So the ERP
> will have nothing left in memory and constantly re-parse the 'sharable' sql
> and go to disk for all the data.  This just doesn't seem logical.
>
> What am I missing here?  How can you possibly have this kind of shared
> environment?  I agree that 'same' type environments may work as pluggable
> databases in a shared SGA, provided you have enough memory, but not
> disparate databases like those in the examples.
>
> I have asked several people, including Oracle instructors the following
> question, but have not yet received a definitive, convincing answer.
> Comments?
>
> --
> http://www.freelists.org/webpage/oracle-l
>
>
>

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