Re: 12c pluggable database shared SGA question
- From: "Kevin Closson" <dmarc-noreply@xxxxxxxxxxxxx> (Redacted sender "ora_kclosson@xxxxxxxxx" for DMARC)
- To: "Glenn.Travis@xxxxxxx" <Glenn.Travis@xxxxxxx>, "Oracle-L@xxxxxxxxxxxxx" <Oracle-L@xxxxxxxxxxxxx>
- Date: Wed, 10 Sep 2014 11:54:29 -0700
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?
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