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sga_max_size (Oracle 10g ONLY)

Oracle Tips by Burleson Consulting
May 7, 2008

Wasting expensive RAM resources is a bad thing and the goal of the Oracle DBA is to maximize the SGA region with intelligent SGA sizing techniques in relation to sga_max_size. 

Beware of AMM:

  • There are reports where automatic memory management (implemented by setting the sga_target and sga_max_size parameters) can cause performance problem, especially for databases with undersized RAM resources. 

  • Oracle (AMM) resize operations can hurt performance.

      In 11g and beyond, Oracle automatic memory management is configured using the memory_target  and memory_max_target initialization parameters.  The memory_target parameter specifies the amount of shared memory available for Oracle to use when dynamically controlling the SGA and PGA. The memory_max_target AMM parameter specifies the max size that memory_target may take.

Note that larger shops many forgo AMM and manually size their shared_pool_size and db_cache_size.   A human DBA can frequently do a better job in sizing the RAM pools, especially when using predictive tuning techniques, which anticipate changes in workload and adjust the pools before a shortage occurs.

If you only have Oracle on the server, start by reserving 10% of RAM for Linux or 20% or RAM for Windows.  With whatever RAM is left-over you allocate to SGA and PGA:

  • sga_max_size -- This 10g parameter sets the hard limit up to which sga_target can dynamically adjust sizes. At database start time, Oracle will allocate sga_max_size in RAM (or set sga_max_size to the sum of the existing pool sizes), so in order not to waste RAM it may be a good idea to have sga_max_size and sga_target at the same value, but there may be times when you want to have the capability to adjust for peak loads. By setting this parameter higher than sga_target, you allow dynamic adjustment of the sga_target parameter.  For related notes, see MOSC Notes 295626.1, 396940.1, 270065.1 and 256913.1.

  • sga_target -- This parameter is for Oracle Database 10g ONLY and reflects the total size of memory footprint a SGA can consume. It includes in its boundaries the fixed SGA and other internal allocations, the (redo) log buffers, the shared pool, Java pool, streams pool, buffer cache, keep/recycle caches, and if they are specified, the non-standard block size caches. 

AIX Note for sga_target:  There are reported cases where sga_target cannot exceed two gigabytes.  This is because of the need to edit edit the /etc/security/limits file and look for Soft DATA segment, it should have the value of -1 (Unlimited).

What should be my sga_max_size?

The default that oracle suggests at the DBCA is 40%, however, the appropriate size for this parameter depends on the space requirements of your actual SGA. Your application requirements will determine this, things like the amount of concurrent users and the activities they are really performing. These types of things can only be monitored when running processes.  There is no formula for an exact value of these parameters.  The best thing you can do is to tune based the advisors suggestions.

Memory consumption should be checked when your real workload is in progress, and sga_max_size tuning should be performed using Enterprise Manager to tune based on realistic metrics.

You can see the Oracle memory segments with the ipcs -pmb command.

 


 

 

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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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