Microsoft Hyper-V 2012

I have several Hyper-V hosts.
Each Hyper-V host has several guest server VMs on them.

I want to do patching on the Hyper-V hosts, and we do not have the option of migrating/moving the guests to other Hyper-V hosts due to lack of disk space.

What is best / safe practice in patching Hyper-V hosts server core?

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You mean you want to patch a host, ie from 2008 R2 to 2012 ??
The most important thing is the hard disc.
The second important the virtual machine definition. (the XML file)

As I don't now about how much volume and VMs we are talking, let me explain the possibilities and the consequences a bit more generic.

The usual way is to export the VM and to reimport it an a new system. You can even use some large USB disc for export / import. The major question is if you can keep the disc, where the VMs are located. If they are on the same disc system like the OS, and you have to destroy it, the only way is to move the VMs to a separate disc system, so export is the option.

If you can keep the disc system because it is independent from the OS, you may bind it back later to the new system and you still have the data.

If export / import is not an option, the simplest way is just to create new virtual machines and to attach the old hard discs (vhd) to the new machines. The lack of this procedure is that you loose the configuration and the NIC. So you have to adjust the VM properties again as well as you have to reconfigure the NIC inside the new VM (will be set on DHCP by default).  If several disc are bound to the VM, you have to be carefully that the disc are bound in the right order to keep the correct drive letters.

You can also take over some settings from the old VM XML file, but not all of them as some of them are specific to the new host.

The new host will produce new GUIDs for the virtual machines as well as for the NICs, and the virtual machines are bound to the new NICs on the new machine. If there are additional virtual devices they are new as well.

So directly taking over the XMLs will not work, you can take over some of the settings if you know which GUID to replace by with new ones. Also the XML files have some specific permissions, which you have to set via power shell in the proper way.

Import / export is the first choice.
The second choice for me would be just to attach the VHDs to newly created virtual machine and to reconfigure them.
zero000koolAuthor Commented:
No I just want to patch the VM host, not upgrade the OS from 2008 R2 to 2012 R2.

I want to run security patches on the VM host.  The individual guest VMs on the Host server have been patched.

Now I want to patch the Host Hyper-V server.
Ok, if you mean just to install some updates....
Usually everything is fine with the exception of the fact, hat some updates forces a reboot.

You can define for every virtual machine what happens in case of a shutdown of the host.
--> Save -  turn off - shut down

If you have dependencies between the machines, you have to take care that the machines are saved, turned off or shutdown in the right order as well as they come up in the right order again. Turn off might not be the best option, so especially data writing servers like SQL may be better to correctly shut down. Save mode is faster but takes some additionally disc space.

If the settings of the virtual machines are fine, and the startup delays are set to avoid that a lot of machines start at the same time, you can just reboot the host and the virtual machines should come up one after each other according to your start delay settings. What are the best values for the start up delay depends on your physical host capabilities.

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