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Hyper V Virtual Machine RAM Limitation

Hello Experts,

We have IBM Physical server with 128 GB RAM installed on it. The server has Windows 2008 R2 Enterprise SP1 OS.
We are trying to assign 100 GB RAM to the Virtual Machine but its not accepting more than 64 GB

Can someone please advise what could be the issue ?

Thanks
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Thanks davorin

Which OS supports more than 64 GB ?
ever tried directly modifying the value (RAM size) in the VM configuration file in plain text format?
Does your Server have two CPU's installed? If so, each CPU is managing 64GB of your total of 128GB RAM (like 2 PC's with 64GB each). It is usually better to use only one socket and then a CPU with more cores in it.

Besides that, it will also depend on your guest OS. Some of them have restrictions about the size of RAM they can use.
@ bing: I have Windows 2008 R2 Enterprise as Guest OS

@ rindi: Our physical server has 4 CPUs and each CPU 6 Cores. We have assigned 16 cores to Guest VM

Thanks
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So I can have Physical OS with Windows 2012 and Windows 2008 R2 as Guest OS?
Thanks Andrew

So how I can assign now 100 GB to guest now.
this is a VM under Hyper-V on Windows 2012?

and you can assign more than 64GB in the VM and power up, and the OS just does not report 100GB ?
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Screenshot of the RAM config might help.  MIGHT.

Also, REDUCE THE CPU!  More CPUs can SLOW DOWN a system.  Unless you really need them, read http://www.zdnet.com/article/virtual-cpus-the-overprovisioning-penalty-of-vcpu-to-pcpu-ratios/
@hancock the VM is on Windows 2008 R2 OS. Its not allowing me to allocate RAM more than 64 GB under VM settings.

@ Which RAM config  you need ?

@ davorin, you are right It seems that I can assign maximum of 64GB of RAM to any single virtual machine running as guests on Windows 2008 R2
We have IBM Physical server with 128 GB RAM installed on it. The server has Windows 2008 R2 Enterprise SP1 OS.
We are trying to assign 100 GB RAM to the Virtual Machine but its not accepting more than 64 GB

That is an OS Hyper-v Version Limitation per Virtual Machine. If you use the free S2K12R2 HyperV or S2K12R2 as the hyper-v host then you don't have the hyper-v VM memory limitation of 64GB (now the limit is 1TB/memory per VM) and you can assign up to the OS memory limitation of the OS that is running inside of the VM.

Server 2016 will have the same limitations as S2K12R2 4TB Host Max, 1TB/VM
Can I upgrade the Host OS to Windows 2012 without reinstalling OS?
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Thanks Bing.

So I can upgrade the server to Windows 2012 Standard
before making such a big move to upgrade the entire OS, just double check if you have ever tried directly modifying the value (RAM size) in the VM configuration file in plain text format?
I never worked with VM configuration file, even if doable, is it recommended ?
obviously NOT recommended for normal operation and administration.

but, it is a manual, ultimate way to configure low-level settings of a VM, something like Windows Registry. by checking the XML file's content in plain text, you can make sure your settings done via GUI are saved as expected, and it is also an essential way to troubleshoot or bypass bugs of Hyper-V GUI, if any.