RE: About having a large SGA for a DataWarehouse

  • From: "Mercadante, Thomas F" <thomas.mercadante@xxxxxxxxxxxxxxxxx>
  • To: "'oracle-l@xxxxxxxxxxxxx'" <oracle-l@xxxxxxxxxxxxx>
  • Date: Mon, 19 Jul 2004 09:05:07 -0400

Hemant,

We currently have a 1 terabyte warehouse.  It's SGA is currently set at
4Gig.  PGA Target at 2500M.  I'm not sure how "big" your users want your SGA
to be.  The real answer (in my opinion) is that it needs to be as large as
it needs to be.  Picking a number out of a hat to start with might be the
wrong approach.  But you have to start somewhere.

If you can post how many users you expect to be querying the warehouse
during the day, we all might be able to help.

Tom Mercadante
Oracle Certified Professional


-----Original Message-----
From: Hemant K Chitale [mailto:hkchital@xxxxxxxxxxxxxx] 
Sent: Monday, July 19, 2004 8:32 AM
To: oracle-l@xxxxxxxxxxxxx
Subject: About having a large SGA for a DataWarehouse



At my site we'll be migrating a  [currently very small 20GB] Corporate 
DataWarehouse
from 32-bit 8.1.7 Solaris to 32-bit 9i Linux.  It will grow rapidly with 
Manufacturing Datamarts
to probably 500GB in a year.

Some of the DataMart teams are pushing for a very large SGA while I would 
rather have a
large PGA_AGGREGATE_TARGET   [not using 10g and Automatic Memory Management]

My rebuttal was :
We must consider how large we really need the SGA.  A too large SGA
[assuming that it is not too large as to cause excessive Paging and begin 
Swapping at the OS level]
can  result in :
   a.  Performance impact in Checkpoints -- larger checkpoints, longer write
times for DBWR
   b.  Performance impact in Free Buffer scans -- longer buffer chains
   c.  Performance impact on Cache Buffer Chains latch -- more buffers per
latch means that the latch may be held more frequently
   d.  Delayed Block Cleanouts -- modified blocks remaining in memory
requiring cleanups and causing potential ORA-1555s


Comments / Recommendations / Corrections from this list, please ?


Hemant K Chitale
Oracle 9i Database Administrator Certified Professional
http://web.singnet.com.sg/~hkchital
"A man's reputation is what other people think of him; his character is 
what he really is."
                                         -- Miner, Jack

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