What does overcommited mean in Virtualization?

Hi Experts,

In XenServer, what does overcommited means? can anyone explain to me in layman's term?

SandManAsked:
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Matt VCommented:
It means you assigned more memory or disk space to the virtual machines that are running than what you have phsyically available on the host server.

So if you have a Vmware server with 4GB RAM and 1TB hard disk, and you create a couple of vritual machines with 3GB of memory each, you will be over committed by 2GB of RAM.
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Carl WebsterCommented:
Very simple to explain.

Most operating systems will rarely use their full allotment of RAM.  If I give a server VM 4GB of RAM, it would be very rare for the full 4GB to ever be fully allocated.  

If my XenServer host has 32GB of RAM and say you leave 4GB for dom0 (or the brains of XenServer) that will leave 28GB of RAM to use for VMs.  If I create 7 VMs and allocate 4GB of RAM to each I have now fully allocated the RAM on the host.  If you were to monitor the 7 VMs, they would probably on average use less the 2GB of RAM.

Knowing this, I could create 12 VMs and allocate each 4GB of RAM or 48GB of RAM allocated to the VMs and 4GB to DOM0 for a total of 52GB of RAM on a 32GB host.  If each of the 12VMs uses on average just 2GB of RAM, the host is actually using just 28GB RAM (12*4 + 4GB for DOM0).

The problem comes in when more VMs need their full 4GB of RAM than the host can provide without having to page VM memory out to the slow hard drives.  Now you have over committed and performance of the host and the VM will suffer.

I personally do not overcommit RAM.  That is just my personal preference.
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SandManAuthor Commented:
Thanks guys for the quick info
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