Re: 12c pluggable database shared SGA question

From: Seth Miller <sethmiller.sm_at_gmail.com>
Date: Wed, 10 Sep 2014 14:05:33 -0500
Message-ID: <CAEueRAWhXudugBvyO-fRDkZVOoG2v-mwr+zEoHMR=W04nsVEGg_at_mail.gmail.com>

  "Shared REDO *should* be the end of the conversation, really."

That's an interesting statement. Please elaborate.

On Wed, Sep 10, 2014 at 1:54 PM, Kevin Closson <dmarc-noreply_at_freelists.org> wrote:

> Shared REDO *should* be the end of the conversation, really.
>
>
>
> ------------------------------
> *From:* Glenn Travis <Glenn.Travis_at_sas.com>
> *To:* "Oracle-L_at_freelists.org" <Oracle-L_at_freelists.org>
> *Sent:* Wednesday, September 10, 2014 9:11 AM
> *Subject:* 12c pluggable database shared SGA question
>
> 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?
>
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> http://www.freelists.org/webpage/oracle-l
>
>
>
>

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Received on Wed Sep 10 2014 - 21:05:33 CEST