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RE: 9i - Dynamic SGA - SGA_MAX_SIZE

From: Kirtikumar Deshpande <>
Date: Sat, 02 Aug 2003 20:09:23 -0800
Message-ID: <>

SGA_MAX_SIZE was introduced in 9i to allow dynamic sizing of SGA (Dynamic Sizing feature) components such as, shared pool, large pool, buffer cache etc. In versions up to 8i, such changes required bouncing the instance.
This parameter assumes the value of the SGA at instance startup. Various components of the SGA can then be increased/reduced as and when needed. The total SGA, thus, can reach a maximum value set by SGA_MAX_SIZE (if set in the init.ora file). That's the idea. However, the implementation is different on various platforms. With ISM, and DISM, on Solaris, there are other issues when it comes to using Dynamic SGA. You may want to search Metalink for specific notes/articles for Solaris.
On AIX 5L as I found out, Oracle uses SGA_MAX_SIZE, if set in init.ora, at the instanace startup, and allocates the excess (difference in computed SGA value and set SGA_MAX_SIZE) to 'variable size'. Hence there is no room for any dynamic sizing (upward) of any SGA component. I did not try to downsize shared pool first, and 'upsize' buffer cache later. May be that would work, but that is not the intention of using this parameter.

PGA_AGRREGATE_TARGET is completely different from this parameter. It sets an instance-wide upper limit for the memory used by sorting, hashing processes.

Hope this helps..

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Author: Kirtikumar Deshpande

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Received on Sat Aug 02 2003 - 23:09:23 CDT

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