RE: 12c pluggable database shared SGA question
- From: "Chitale, Hemant K" <Hemant-K.Chitale@xxxxxx>
- To: "ORACLE-L" <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 11 Sep 2014 10:52:14 +0800
Imagine only 2 PDBs in 12c.
PDB "A" generates 1GB of redo per day.
PDB "B" generates 100GB of redo per day.
All is fine when running normally.
Now imagine having to do a PITR recovery of PDB "A" as of 1.5 days ago.
Unless you have frequent Incremental Backups of PDB "A" you'd have to
have Oracle go through a lot of archived redo that is "thrown away" to
be able to do the PITR for PDB "A".
Extend this to PDB "A" being one of 24 PDBs in the same MultiTenant
environment.
Hemant K Chitale
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Kevin Closson
Sent: Thursday, September 11, 2014 2:54 AM
To: Glenn.Travis@xxxxxxx; Oracle-L@xxxxxxxxxxxxx
Subject: Re: 12c pluggable database shared SGA question
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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