Required physical server Vs Virtual Machine memory

Is there a white paper explains why we we need less memory in the Virtual Machine as compared to the physical Machine.

The problem is that my boss wants to know why this is so. It is very difficult for me to explain this to him, he wants to see a white paper OR some article on the internet.

As long as a VM administrator, I know that we can add and remove a virtual computer memory but it will be not that simple when it comes to physical servers. I suggested to him that we must follow a standard, and if there is a requirement for business we can increase or decrease is according to the needs. So when the salesman comes in they give us specifications for the physical server and not the virtual and there is always an argument that why should have 6-8 GB memory VM from the outset. In order to avoid everything that the arguments it needs more proof.

Thank you

Here is a simple explanation of why VM need less memory.  Transparent page sharing.

If you install a virtual of Windows 2008 SP2 single server machine, lets assume that it uses 500 MB of RAM while sitting idle.  It's basically all running components loaded into memory.

If you then put a same VM, the new machine virtual don't re-charge not the exact same components in memory.  ESX will see the same exact memory block has been loaded into the memory of the host Server1 (phyiscal material) and then simply point the virtual machine to the component already loaded into memory.

After the virtual machine has been loaded and GST has kicked, you will probably find that for 2 identical virtual machines you use not only 650MB of memory (not 1000 as expected).  This is because 80% of the components has need of the new virtual machine, it only uses the existing memory blocks, instead of duplicating.

Now, repeat this operation on 10 VM and assume only 50% of the memory is the same between them... it is still a huge economy of memory.  In this case, since each VM Windows is really loading only 250 MB of itself in memory and split the rest, technically you don't need to allocate 2 GB of RAM for each virtual computer... you can easily get away with putting 1 to 1.5 GB on this server and I have always the same ratio of available memory for the programs that you would do if it was a physical server.

Tags: VMware

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