Re: 12c pluggable database shared SGA question
- From: Paresh Yadav <yparesh@xxxxxxxxx>
- To: dmarc-noreply@xxxxxxxxxxxxx
- Date: Wed, 10 Sep 2014 15:19:58 -0400
Assuming current SGA size is already optimized for current data sets, won't
additional PDBs with their own data to be cached need more buffer cache and
more SQL statements need more memory in library cache? What is than meant
by "....only *small increments* of memory are added as additional PDBs are
added."? There is a fixed area in SGA which is quite small, is that what is
being referred by savings in memory between PDBs?
Thanks
Paresh
416-688-1003
On Wed, Sep 10, 2014 at 2:54 PM, Kevin Closson <dmarc-noreply@xxxxxxxxxxxxx>
wrote:
> Shared REDO *should* be the end of the conversation, really.
>
>
>
> ------------------------------
> *From:* Glenn Travis <Glenn.Travis@xxxxxxx>
> *To:* "Oracle-L@xxxxxxxxxxxxx" <Oracle-L@xxxxxxxxxxxxx>
> *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?
>
> --
> //www.freelists.org/webpage/oracle-l
>
>
>
>
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