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Estimating SGA size tips

Oracle Database Tips by Donald Burleson

Estimating SGA size is critical for all capacity planning activities and estimating the SGA size ensures that you do not waster precious and expensive RAM.  The goal of server optimization for any Oracle databases is to manage the RAM and CPU resources of the machine, and make sure that expensive RAM is not under-allocated.

See here, full method for estimating SGA size.

 

Estimating Oracle SGA size

The Oracle DBA can use math to determine the optimal RAM allocation for a MS-Windows server. For the purposes of this example, let's assume that we are on a dedicated MS-Windows Oracle server, and Oracle will be the only program running on the server. The total RAM SGA and PGA estimates for Oracle are as follows:

  • OS RAM :   20 percent of total RAM for MS-Windows, 10% of RAM for UNIX
     
  • Oracle SGA RAM :   determined with the show sga command
     
  • Oracle database connections RAM :   Each Oracle connection (when not using the Oracle multi-threaded server) will use two megabytes of RAM.  Should a task required sorting or a hash join, the RAM used is set by sort_area_size and hash_area_size (if not using pga_aggregate_target).

Once we know the total available RAM memory, we have to subtract 20 percent from this value for MS-Windows overhead. Even in an idle state, Windows services use RAM resources, and we must subtract 20% to get the real free RAM on an idle server.  This serves as a baseline estimate of SGA size.

See my related notes on estimating SGA size for Oracle here:

 

 
If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

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