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# Core Server - minimum space

Posted on 2011-03-04
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How do I calculate the RAM needed by a Core Server running HyperV VMs? Is it dependent on the RAM's consumed by the VM's hosted on it? I will give a couple of scenarios to illustrate the point.

Scenario 1:

Server 1 - 16 gig of physical RAM

VM 1 - 6 gig

VM 2 - 4 gig

Does this mean I have fully 4 gig of RAM left for the Core or is there some overhead of some kind e.g. paging for each VM which then means less than 4 gig left for the Core?

Scenario 2:

Server 1 - 16 gig of physical RAM

VM 1 - 6 gig

VM 2 - 4 gig

VM 3 - 5 gig

This would leave 1 gig for the Core but because I have more RAM in total and more VM's does this impact how much RAM the Core has for itself?

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Question by:lineonecorp
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Felix Leven earned 500 total points
First Game has changed on Hyper-V since Sp1, because of the DynamicMemory Function.
http://technet.microsoft.com/en-us/library/ff817651(WS.10).aspx

At the end of this Article you find an appendix listing the recommended Startup RAM for supported Servers in Hyper-V.

just to be sure the new memory feature won't take away to much memory from the parent host (?1024MB? standard), you can set:
HKLM\Software\Microsoft\Windows NT\CurrentVersion\Virtualization
to 2048 for example,
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Felix Leven earned 500 total points
sry

HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Virtualization\MemoryReserve
decimal REG_DWORD
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Author Comment

Interesting but what I am more interested in is how much memory do I need to leave for the Core and how that's related to the number of VM's and the RAM they've been allocated. Is 1 gig of RAM left for the Core in example 2 or do I need more? Would I need more left of the Core if I had 32 gig of physical RAM and 31 gig in guest VM's? Or will 1 gig or 2 gig do no matter how much physical RAM I have and how many VM's are consuming it?
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Felix Leven earned 500 total points
i don't think you need more than 1 G of ram left for the parent host in case you install just three servers (or 10-15 Servers). care more about the sizing of the VM you will install. what services will be installed, mail, db, webserver. do you need a second server for live-migration in case of an upgrade to the server hardware/desaster recovery.
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Felix Leven earned 500 total points
memory overhead of each VM is 32 MB for the first 1 GB of virtual RAM plus another 8 MB for each additional GB of virtual RAM + 512MB ParentHost

MS Performance Guides now include Hyper-V:
http://msdn.microsoft.com/en-us/windows/hardware/gg463392
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Author Comment

So if I have 5 VM's each with 3 gig of virtual RAM  I would need:

32 meg for the 1st gig of virtual  RAM for each VM  * 5 VM's = 160 meg of VM overhead RAM

8 meg of RAM for each gig of  the remaining 2 gig of RAM in each VM for a total of 16 meg for each VM times 5 VM's = 80 meg of VM overhead RAM

So total overhead RAM in this scenario would be 240 meg of RAM

I would also need 512 Meg for the host Core server

So in total in this scenario I would need 752 meg of RAM above and beyond the 15 gig of RAM for the VM's.

So 16 gig of RAM in this type of system would be adequate?

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Felix Leven earned 500 total points
Yes, 512 mb hyper-v + recommended memory for the server edition you are using, but with the right configuration of dynamic memory you can free more memorye for future use. But keep in mind This is the theoretical view, for standard workloads (services your vm's offer).
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Author Comment

Sorry not following.  Were my figures correct?  752 meg of RAM above and beyond the 15 gig of RAM for the VMs?
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Felix Leven earned 500 total points
Yes, there will be some small tweaking here and there but it will be ok.
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Author Comment

Thanks.
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