How many virtual machines are there on one Hyper-V server?

The question that virtualization administrators often face is: How many? To steadily predict your data center and your budget, you always want to know how many virtual machines can be merged on one physical machine. The answer is always the same: depending on the situation.

In the era of physical chassis, it is certainly much easier to propose this number. When facing an application, you are advised to configure each application, which usually consumes more energy than the increase. Currently, this space is shared by other virtual machines. The appropriate quantity depends on the machine type, their workload, and requirements for RAM, storage, and CPU. The daily expenses for managing complex and multi-threaded workloads also affect the final quantity. With virtualization, all content is related to the efficient use of resources.

When considering virtual machines, it is still a good idea to start with your processor needs. Although many or even the most basic models currently on sale have multiple processor cores, this does not mean your servers need them. You can allocate four virtual processors to a virtual machine based on the operating system, but synchronizing multiple physical processor cores to serve the configuration of a multi-CPU virtual machine can indeed reduce daily expenses. The decline in integration leads to an increase in daily spending. Cross-processor communication issues less in Windows Server 2008 R2 virtual machines than in Windows 2000 Server. You can crop the number of virtual processors to provide only the resources required by virtual machines.

The number of virtual processors that can run on a single server is limited. Microsoft Hyper-V Server R2 will support up to 512 virtual CPUs on each physical host. This is a large number, but it does not mean that you can reach this number without any consequence. Different software vendors may have different requirements on their CPU, but you should eventually think about how many virtual CPUs are running on each logical CPU, as well as the proportion. Unlike some virtualization platforms, this is not the core, but the core and thread. Related examples are hyper-Threading Technology Used in most Intel Server chips. Hyper-threading is the reason why Windows reports eight CPUs when there is a full CPU core. In this case, you can calculate the ability to execute two threads in a clock cycle, because hyper-threading allows you to do this. In this case, you usually double the actual physical core of the server.

So how do you evaluate your CPU needs? It is generally recommended to allocate Eight virtual processors for a single logical processor for the server, and assign 12 virtual processors for each logical processor for the virtual desktop, such as Windows 7 virtual machines. On Hyper-V hosts running two six-core HT processors and server-level virtual machines running a single virtual processor, you may limit the number of virtual machines to less than 192.

The current server processor performance has made a huge leap in virtualization. Each road can reach up to six cores, as well as optimization and faster bus such as the growth of level-2 and level-3 caches. Now the server can handle more problems than before. In a recent conference, Microsoft announced that each of their laboratory's server blades contains 225 virtual machines with 5600 GB of memory and two Intel Xeon processors, each host has six cores. This density is not even because the workload is close to the CPU and memory capacity, and the CPU utilization is generally around 15%.

This is okay when talking about basic servers, but what will happen when the server runs a CPU intensive load? You can allocate additional CPU cores to a virtual machine. For example, SQL Server requires not only a processor, but also a lot of memory. In this case, the memory configuration you may run is limited to 64 GB per virtual machine. Although it is common to push virtual machines to lightweight, dual-path hardware, Hyper-V does support up to 64 Logic processors on the host, which allows you to scale out the solution for applications that urgently need CPU.

This is also true when considering the higher limits of virtual machines. If you are on a cluster Hyper-V host, the maximum number of virtual machines you can run on one host is 384, however, the total number of VMS used for clusters is limited to 1000. This is also the case when you use large memory for virtual machines. Remember that the total memory available for the Hyper-V host is 1 TB, so plan your VM allocation based on the actual situation.

Some changes have been made in the latest Windows Server 8 announcement. Although this is not final yet, Hyper-V is expected to suddenly upgrade the virtual processor from 4 to 16 CPUs, this allows you to allocate specific workloads to specific cores. If you have long-term plans and intensive processor workloads, you will want to consider how these will affect VM allocation within one year from now on.