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.


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]  vs. [12c]
> 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?
> --

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