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Don Burleson Blog 







sga_size_factor Tips

Oracle Database Tips by Donald BurlesonMarch 27, 2015

Question:  I need to understand the sga_size_factor column of the v$sga_target_advice and dba_hist_sga_target_advice.

Answer:  The sga_size_factor column of dba_hist_sga_target_advice (an AWR history table) shows changes to the size of the SGA, as constrained by the sga_target (10g) or the memory_target (11g and beyond) parameter.  This is valid only when using automatic memory management (AMM).

For example, in the query below show those time when the SGA size did not change as a result of an automatic memory management (AMM) resize operation:

   sga_size_factor = 1
order by
   snap_id asc;

---------- ----------
4           508
5           508
6           508
7           508
8           508
9           508
10          508
11          508

Within v$sga_target_advice, the sga_size_factor, show the relative changes to SGA performance for various sizes of the SGA, ranging from 50% of sga_size_factor=1, up to double the value (200%).

order by
   sga_size asc;

292 0.5 2.3423
438 0.75 1.192
584 1 1
730 1.25 0.5634
876 1.5 0.5634
1022 1.75 0.5634
1168 2 0.56345

This data is used with AMM (a.k.a. (Automatic shared memory management (ASMM) to determine whether you want to increase sga_target (memory_target) to improve performance within the SGA.

Fir large mission-critical databases, many DBA's will not use AMM and prefer to allocate and morph the pools manually.  This allows a more intelligent approach to SGA tuning than Oracle's AMM algorighm.

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