Impact of resetting sga_target

We've just made changes to increase memory and implement hugepages.  One of the objectives was to also increase db_cache_size to 26G, but got an error saying there was not enough memory.  We have 64G memory with 42G in sga and 7G in pga.  in our parameter file i have both sga_max and sga_target.  I see someone that got the same error resolved the problem by setting sga_target =0 then alter system set db_cache_size to desired size.  
My question is since the application is up, what's the impact of setting sga_target to 0 while there are users on the system?
xoxomosAsked:
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slightwv (䄆 Netminder) Commented:
From your last few previous questions I know you are working with a Vendor to troubleshoot what I believe to be performance issues.

Any reason you are deviating from Automatic Memory Management?

http://docs.oracle.com/cd/E11882_01/server.112/e25494/memory.htm#ADMIN11011

As for setting sga_target to 0:
http://docs.oracle.com/cd/E11882_01/server.112/e40402/initparams052.htm#REFRN10033

      If SGA_TARGET is set: If the parameter is not specified, then the default is 0 (internally determined by the Oracle Database). If the parameter is specified, then the user-specified value indicates a minimum value for the memory pool.

If SGA_TARGET is not set, then the default is either 48 MB or 4 MB * number of CPUs, whichever is greater
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xoxomosAuthor Commented:
When we startup each week after cold backup we get this advisory in the alert log telling us before we start up again to implement hugepages. Looking at directions on implementing huge pages i see:
"... Automatic Memory Management (AMM) feature which is incompatible with HugePages. Disable AMM before proceeding. To disable, set the initialization parameters MEMORY_TARGET and MEMORY_MAX_TARGET to 0 (zero)."
I think I phrased the question poorly again:-)  Our current situation is we just implemented hugepages early this morning and increased other parameters.  We intended to increase db_cache_size to 26G, but got an error saying there was not enough memory.  Total is 64G, up from 16.  SGA 24G up from 8G and PGA 7G up from 1 1/2G  so there is enough memory.
I saw one of those dba-oracle guys had a similar problem that he resolved by setting sga_target to 0 then setting db_cache_size.
SQL> alter system set db_cache_size = 1024M;
alter system set db_cache_size = 1024M
*
ERROR at line 1:
ORA-32017: failure in updating SPFILE
ORA-00384: Insufficient memory to grow cache
Even then, it would not let me grow the buffer cache. I had to disable sga_target for 11g to finally allow it:

Solution

SQL> alter system set sga_target = 0;
 
System altered.
 
SQL> alter system set db_cache_size = 1024M;
 
System altered.
 
SQL> alter system set memory_target = 1648M;
 
System altered.
 
Right now students are banging away taking finals.  
What i am really asking is if i go in and try setting sga_target to 0 on the fly, is there ANY chance it would even for a second interfere with our long suffering students?
I'm going to post another question because despite the fact that I had to settle for 16G for db_cache_size in my initfile, when i query v$sgainfo, it's saying db_cache_size IS 25,728 (~26G) , the value that i got the error on when i tried to startup with that in the initfile.
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slightwv (䄆 Netminder) Commented:
The only possible issue setting sga_target to 0 is that performance can suffer if Oracle cannot get enough memory to run efficiently.

I've seen 'not enough memory' memory errors on startup when I knew there was because Oracle needs contiguous memory when it goes to get it.  There was an issue when I would shutd down Oracle and immediately try to start back up memory would become fragmented.

Only a serer reboot would allow me to start up the database.
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