Oracle-L: RE: 9i - Dynamic SGA

From: Roger Xu <roger_xu_at_dp7uptx.com>
Date: Tue, 05 Aug 2003 15:29:23 -0800
Message-ID: <F001.005C912B.20030805152923@fatcity.com>

Why I have db_cache_size set to zero in my database?

SQL> show parameter db_cache_size

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_cache_size                        big integer 0

SQL> -----Original Message-----
Sent: Tuesday, August 05, 2003 5:34 PM
To: Multiple recipients of list ORACLE-L

don't know if it's still true, i played with sga_max_size and db_cache_size a year ago on 9.0 & Solaris8 & F15k w/o much luck.

when sga_max_size = 64G and db_cache_size = 32G, db started impressingly fast. but db server cpu sys% jumped up to 90+ during warming-up tests, leaving usr% close to 0. tests never finished. reduced sga_max_size = 40G and db_cache_size = 32G, db cpu still around 50% sys. sga_max_size = 18G and db_cache_size = 16G, db performing ok. after that gave up sga_max_size, increased db_cache_size to 24G (sga_max_size was then auto set to 24G). db performing ok. db_cache_size = 32G (= sga_max_size). took 32 min to start db. db performing ok. db_cache_size = 40G (just curious), took 40min to start db. db performing ok.

-Jessica

Mohammed Shakir <mshakir08816_at_yahoo.com> wrote:
> Kirti
>
> Thanks for the info.
>
> I could not raise dynamically the size of db_cache_size and did not
> know why, until I noticed the new parameter sga_max_size. Anyway, I
> tried to lower the size of db_cache_size dynamically and I did not
> have a problem. So sgma_max_size does play its role in
> db_cache_sizing.
>
> I have not tested where I increase the size of sga_max_size using
> init.ora and then try to increase the size of db_cache_size
> dynamically by the same size as the increase in sga_max_size. That is
> the test for coming weekend.
>
>
> --- Kirtikumar Deshpande <kirtikumar_deshpande_at_yahoo.com> wrote:
> > 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..
> >
> > - Kirti
> >
> >

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Received on Tue Aug 05 2003 - 18:29:23 CDT

RE: 9i - Dynamic SGA - SGA_MAX_SIZE