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As servers requires large amount of main memory can I have a Machine with 20GB of Secondary Memory(Hard Disk) and 80GB of Main Memory(RAM). Are their any rules of mapping Main Memory and Secondary Memory in a Machine.

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Hard drives are storage, not memory. –  ultrasawblade Aug 2 '13 at 13:32

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up vote 8 down vote accepted

To answer your question directly, it is perfectly fine to have more memory than hard drive storage.

RAM and storage are completely independent of each other. The amount of each is irrelevant. Well, thats not entirely true. You need some RAM for a computer boot. However, a hard drive (permanent storage) is not necessary for a computer to function. Remember, computers way back when, didnt even have hard drives, floppies, or any permanent storage.

There are some disadvantages in having a smaller hard drive than memory with modern operating systems. Paging, hibernation, and memory dumps, just to name a few. However, they are points to consider, but in no way will prevent you from running anything.

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I don't even think the 'common' formula for determining the page file size applies in this case. –  Maurice Stam Aug 2 '13 at 13:19
    
And as for the specific numbers in the question: 20 GB is not enough for a Windows Vista or later installation when the updates start coming in. I have had XP virtual machines running for years that could do with 20, but later Win versions bump against that barrier easily (don't know the min required size yet). Linux etc I would not know. –  Jan Doggen Aug 2 '13 at 13:31
    
@Aphelion The 'common' formula (2xRAM) still applies. The purpose is such that the OS can keep a copy of everything in memory also in the page file. This makes it much faster to dump cache to make room for new data since the OS can simply reuse the memory pages and doesn't have to write them out to disk first, which is painfully slow. If they're already on disk, then cache memory is always available. –  Darth Android Aug 2 '13 at 13:34
    
Modern Windows would not fit on 20GB - not without some modification. However, there are small *NIX OSs that are designed to fit on tiny drives (4GB or less). –  Keltari Aug 2 '13 at 13:35

Harddisk dos noet equal memory.
I think you mean SWAP memory which is stored on the harddisk.

As Keltari already said: RAM and Harddisk are independent of each other.

And SWAP is also not a requirement, although little bit of swap is for most operating systems recommended, even if there is plenty of real RAM to go around.
(E.g in many Unix/Linux systems the swap area is also used to save debugging information if the system crashes.)

In the old days (20 or more years ago) there was a rule of thumb that you need SWAP to be 2-3 times bigger than RAM.
That is completely nonsense in modern computers, but there are a lot of people still repeating that old rule-of-thumb as if it is God's own truth.

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