FAQ by John Savill at Windows IT Pro

Q: Is it OK to create a Hyper-V virtual machine (VM) with an odd number of virtual processors?

A: A VM can have between 1 and 64 virtual CPUs in Windows Server 2012 and later. Typically, VMs are given 1, 2, 4, 8 and so on virtual processors. This matches many physical server configurations.

Although 1 is an odd number, it's not common to see a VM with 3 or 5 virtual CPUs. However, there is nothing to stop you from doing that. Hyper-V and Windows itself have no problem with an odd number of processors, and there's no need to have an even number.

Q. I notice that different guest OSs have different numbers of virtual processors supported with Hyper-V. What happens if you try to assign more than the supported number of virtual processors?

A. Hyper-V in Windows Server 2008 R2 supports up to four virtual processors per virtual machine (VM), but different guest OSs running within those VMs are tested with different numbers of processors—not all guest OSs are supported for use with the full four virtual processors. The full list of OSs and supported number of virtual processors can be found on this Microsoft site.

If you install a guest OS that, for example, is listed as only supporting two virtual processors, but you configure the VM with four virtual processors, it will still boot—there's no hard limit. However, the configuration wouldn't be supported, and there might be valid performance reasons why more virtual processors aren't supported, so you might see poor performance. But there's nothing that would actually stop you.

Q: Can I perform a Live Migration from a Hyper-V host with an Intel processor to a Hyper-V host with an AMD processor in Windows Server 2012?

A: Live Migration allows a virtual machine (VM) to be moved from one Hyper-V host to another without any downtime to the guest OS. A processor compatibility feature allows a VM to be moved between Hyper-V hosts that have different versions of the same brand of processor, such that both servers have Intel or both have AMD-based processors. However, Live Migration even with processor compatibility enabled on the VM can't be used to move between a mix of AMD and Intel servers. Given how different AMD and Intel processors are, the ability to move a VM between servers running AMD and Intel is fairly unlikely.

Q: Is there a hard limit on the number of Hyper-V Replica servers that can be enabled from a specific Hyper-V server?

A: There's no hard limit for the number of Hyper-V Replica servers that can be enabled from a Hyper-V server or to a Hyper-V server. However, you should carefully consider the following when planning the number of Hyper-V replicas and the infrastructure required.

First, for each Hyper-V replica there's additional write activity on the volume containing the virtual hard disks (VHDs) of the virtual machine (VM) for the log files. The write activity isn't doubled, as there are many optimizations used in the log creation. However, it still increases, around 20 percent is fairly typical from my tests, so this would need to be considered and would vary based on the characteristics of the workload running inside the VM.

Second, and still related to storage, is the actual space the log files consume. Normally the log files are sent to the replica every five minutes. After confirmation that the log file has been played into the replica VM, the log file is deleted from the primary. However, if there's a large amount of change on the source VM (also called churn), the log file could grow large, and any interruption in the transmission of the log file would result in multiple log files queuing up on the source server, consuming more disk space.

Third, the log files have to be sent over the network, and depending on the workload of the VM, the log files could be large and require network bandwidth to be sent. The more VMs enabled for Hyper-V Replica, the more bandwidth required. If the network bandwidth between the primary and the replica isn't sufficient, the log files will start to queue up and the replica will start to creep further out of synchronization.

Finally, and a much smaller consideration, Hyper-V Replica will use some additional CPU and memory. However, I haven't seen this to be a big consideration, and the disk and network factors would be more important.

Q. Is the eight virtual processor per logical processor limit in Hyper-V limit enforced?

A. When calculating resource capacity, you should ensure you don't exceed the recommendation of eight virtual processors to each logical processor. Remember that a logical processor is a core, so if you have a quad-core processor, it counts as four logical processors, so up to 32 virtual processors would be supported.

This isn't an enforced limit, so in theory you could exceed eight virtual processors per logical processor. However, if you hit a problem, it would not be supported by Microsoft, and it really isn't recommended. Proper capacity planning is unlikely to recommend more than eight virtual processors per logical processor unless you have extremely light loads.

Q. How do I set a CPU reservation for a Hyper-V virtual machine (VM)?

A. Hyper-V allows you to reserve processor resources for a VM in addition to limiting the amount of processor resource usage. To do so, access a VM's settings and select the Processor node.
Initially, you select the number of logical (virtual) processors that the VM can see, from one to four (assuming the physical host has at least four cores). You can then set various Resource control options, as shown below.

Reference: John Savill | Windows IT Pro| http://savilltech.com/blog/