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With Multiple Instances Do I need to change shminfo_shmmax kernel value on Solaris 9?

Posted on 2007-10-04
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I am running Oracle 9i on Solaris 9 Sparc platform. Our server (Sun Fire E20K) has 64 Gigs of ram. We have 6 different instances of Oracle running. The sga_max_size for each database is set to 5 gigs. Our shmmax kernel setting is set to around 6 gigs. Should the shmmax be larger? In particular someone said that the shmmax kernel parameter should be set to the sum of the sga_max_size of the instances we have running. Is this true? I have thus been unable to find any documentation on this.

Thank you
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Question by:umfkit
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joebednarz earned 1000 total points
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From Oracle Configuration Guide for Solaris:

SHMMAX -- The maximum allowable size of one shared memory segment. The SHMMAX setting should be large enough to hold the entire SGA in one shared memory segment. A low setting can cause creation of multiple shared memory segments, which may lead to performance degradation.
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by:schwertner
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Of course yes!
You have to ensure the proper values for all kernel
parameters as they are mentioned in the
Oracle Installation guard.

The SHMMAX parameter is used to define the maximum size (in bytes) for a shared memory segment and should be set large enough for the largest SGA size. If the SHMMAX is set incorrectly (too low), it is possible that the Oracle SGA (which is held in shared segments) may be limited in size. An inadequate SHMMAX setting would result in the following:
ORA-27123: unable to attach to shared memory segment
You can determine the value of SHMMAX by performing the following:

# cat /proc/sys/kernel/shmmax
33554432
As you can see from the output above, the default value for SHMMAX is 32MB. This is often too small to configure the Oracle SGA. Generally it is recommended to set the SHMMAX parameter to 2GB.

Also I think that allowing SGA to grow to 5 GB as you mentioned is overhelming.  Do you really
need so big SGA? There are issues with too big SGAs.
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Computer101
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