RE: 12c pluggable database shared SGA question
Date: Wed, 10 Sep 2014 19:29:24 +0000
Message-ID: <dd64a177fe324f0f845ade3b1ec74a14_at_MERCMBX44D.na.SAS.com>
I was hoping you could tell me. That statement from straight from the promotional materials.
The other question raised is that if you add more (ample) memory for the new PDB, what’s to prevent the other PDBs from utilizing it?
From: Paresh Yadav [mailto:yparesh_at_gmail.com]
Sent: Wednesday, September 10, 2014 3:20 PM
To: dmarc-noreply_at_freelists.org
Cc: Glenn Travis; Oracle-L_at_freelists.org
Subject: Re: 12c pluggable database shared SGA question
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_at_freelists.org<mailto:dmarc-noreply_at_freelists.org>> wrote: Shared REDO *should* be the end of the conversation, really.
From: Glenn Travis <Glenn.Travis_at_sas.com<mailto:Glenn.Travis_at_sas.com>> To: "Oracle-L_at_freelists.org<mailto:Oracle-L_at_freelists.org>" <Oracle-L_at_freelists.org<mailto: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-lReceived on Wed Sep 10 2014 - 21:29:24 CEST