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RE: 9i - Dynamic SGA - SGA_MAX_SIZE

From: Venu Gopal <venu.andem_at_wipro.com>
Date: Fri, 01 Aug 2003 03:14:24 -0800
Message-ID: <F001.005C8376.20030801031424@fatcity.com>

Stephen,

That's the way it's meant to work.
This is how Oracle guarantees that you can grow upto the value of SGA_MAX_SIZE. Look at the case below:

DAY 1: you have your sga_max set to 100m but you are currently using only 50m of it.
DAY 2: you want to use all 100m and you try to increase the sga size, but some other app has used rest of the physical memory and there is no sufficient memory available, at this stage oracle has to error out saying 'insufficient memory', but in reality it does not!!

In reality: On DAY 1, Oracle tries to occupy all 100m in the memory so that it doesn't have any issues in future... this way you have 100m reserved for oracle, use it whenever you want to use it.

Hope this helps you...

~Venu

-----Original Message-----
Stephen.HODGKINSON_at_eaguk.com
Sent: Friday, August 01, 2003 3:24 PM
To: Multiple recipients of list ORACLE-L

Hi,

does anybody have any experience with setting the SGA_MAX_SIZE in 9i.

I assumed the purpose of this parameter was that SGA would grow as requested to that limit.

Example:
You could configure your SGA to be 80M
Set the SGA_MAX_SIZE to be 250M.

I would have expected oracle to acquire 80M of memory from the UNIX machine.

In fact using ipcs you can see that oracle will always acquire the value
of SGA_MAX_SIZE.

It acquires the extra space in the Variable Size of the SGA          

 Sga_max_size=75m.  

 Show sga:

 Total System Global Area 77041728 bytes

 Fixed Size 733248 bytes

 Variable Size 41943040 bytes

 Database Buffers 33554432 bytes

 Redo Buffers 811008 bytes    

 sga_max_size=110M and keep everything else the same.  

 show sga:

 Total System Global Area 114790680 bytes

 Fixed Size 733464 bytes

 Variable Size 79691776 bytes

 Database Buffers 33554432 bytes

 Redo Buffers 811008 bytes

 Database mounted.

 Database opened.    

I have raised a lengthy call on Metalink and the consultants are convinced
this is normal behaviour and what you would expect.

Do people agree with the metalink consultants?

Maybe my expectations were to high but I thought a dynamic sga would mean I
could change the amount of memory acquired by the UNIX box.

All opinions welcome. I am on tru64 platform - 9.2.0.3.0

Thanks, Stephen



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Author: Venu Gopal
  INET: venu.andem_at_wipro.com
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