Part of the confusion DBAs face stems from the fact that physical memory is used for a variety of purposes and classified differently. There is real memory, virtual memory, shared memory, nonshared memory, private memory, shared memory segments, code, data, stack, resident memory . . . I'm sure there are other types, names, and categories. The confusion can be significantly eliminated by grouping memory into three categories: real and virtual, shared memory segments, and process-related memory.

The real and virtual memory category is pretty simple. Real memory is the actual memory chips. Virtual memory is not real but provides the appearance of a lot more real memory. Some operating systems manage this better than others, but I digress.

Shared memory segments reside in real memory (we hope), and a process creates them using the shmget call. Most DBAs quickly figure this out once they try to start an Oracle instance and receive a shmget-related message, and then need to track down what the heck just happened.