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If I have 8Gb RAM and 8Gb swap and an application requests 20Gb will it run out of memory?
No: The request will be refused.
Btw. The old rule that you must have at least as much swap as you have memory does not apply to modern unix.
Many modern unix systems can be configured to use part of memory for swap. This allows the O/S to defragment main memory quickly and quietly.
If you have Oracle, buy enough memory so that it should never swap and be sure that you understand how to size the SGA and PGA and how to measure these sizes using Oracle itself. It is very common to missize the SGA by a factor of 8 or even 16.