What is SGA_TARGET And LOCK_SGA?

Question

Thanks & Regards
vijay

Answer 1

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So, my question is what will be effect of "alter system flush" if LOCK_SGA is set to TRUE....
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It will do the same thing it always does. See the FLUSH SHARED_POOL Clause
http://docs.oracle.com/cd/B28359_01/server.111/b28286/statements_2013.htm
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The FLUSH SHARED POOL clause lets you clear all data from the shared pool in the system global area (SGA). The shared pool stores
•Cached data dictionary information and
•Shared SQL and PL/SQL areas for SQL statements, stored procedures, function, packages, and triggers.
This statement does not clear shared SQL and PL/SQL areas for items that are currently being executed. You can use this clause regardless of whether your instance has the database dismounted or mounted, open or closed.
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Logically the SGA can be considered one monolithic block of memory; Oracle knows what is in it but to the OS it is opaque. The entire SGA might be in memory or part of the SGA may be in memory and part may have been 'swapped' to disk by the OS.
In either case the OS does not know and does not care what is in the SGA. If it needs memory for other things it may swap (page) part of large memory segments to disk and then if a 'memory' reference is made to a part that is on disk the OS will load it back into memory and may swap something else out to disk to make room for it.
LOCK_SGA ensures that all of the SGA is kept in memory and prevents any of it from being swapped to disk.
Flushing is an Oracle process that flushes the 'contents' of the SGA regardless of where the SGA is physically located. The part in memory and any swapped parts will all be flushed. The flush process does not know, and does not care if all of the SGA is in memory or if part of it is swapped out.
They are two separate and distinct operations.