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SolutionBase: Considerations for running multiple virtual machines in Virtual Server 2005 R2

Virtual Server 2005 R2 allows you to host other operating systems on your server. Things get tricky when you want to run multiple virtual machines. Here's what you'll encounter.
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Virtual Server 2005 R2 (VS2005 R2) gives network administrators an easy way of consolidating multiple servers and OSs on to a single computer. Instead of using a dedicated server for every application and service you require, you can run the applications and services on virtual server instances. This allows you to continue to isolate applications and services while making better use of existing hardware.

VS2005 R2 can support up to 64 virtual machines (VMs) on a single host. In reality, however, the number of VMs you can successfully run on one host will depend on the physical hardware -- specifically the amount of RAM and available hard drive space. In this article, I'll talk about some of the aspects you need to consider when deploying multiple VMs under VS2005 R2. I'll also discuss any requirements unique to running multiple VMs.

System requirements

The number of VMs you plan to run simultaneously on a host will have an impact on the host computer's system requirements -- specifically on the amount of RAM and hard drive space required.

Memory

Memory is critical to successfully run multiple VMs on a single host. The minimum requirement to install VS2005 R2 is between 256 and 512 MB, depending on the host platform you are running. These values, however, are just starting points. If you plan to run more than one guest OS simultaneously, you need to begin increasing the RAM. A general guideline to follow is to add an additional 32 MB of RAM for each VM.

Note: When calculating the amount of RAM, don’t forget to factor in the RAM required to run host OS.

Disk space

Adequate disk space is just as important as adequate RAM, since each VM will have its own page file and dynamically expanding virtual hard disk. Additional space may be required on top of this if you plan to put your VMs into a saved state. VS2005 R2 requires a minimum of 2 GB free disk space. This amount will therefore need to be adequately increased for each VM you plan to install.

Multiple processors can improve the performance of the host OS and Virtual Server 2005. However, guest OSs themselves can not take advantage of multiple processors.

VM settings

If you are running a single VM on your host, you don’t have to be too concerned about the settings. It's when you get into a multiple VM configuration that you need to start thinking about them, because some of the settings can affect all the VMs running on a host, not just one. Let’s take a look at some of the settings you need to pay close attention to.

File name

When you create a new VM, you are prompted to provide a file name. This is the name assigned to the VM configuration (.vmc) file. This file name can not exceed 150 characters and if you are running multiple VMs, each one must have a unique file name.

CPU Resource Allocation

You need to pay close attention to the CPU Resource Allocation settings when running multiple VMs. For example, if one of your VMs will not start, it may be due to over-allocated CPU resources. In other words, some VMs have been assigned more CPU resources than others.

Since only those VMs that you have permissions to appear on the CPU Resource Allocation page, it is easy for someone to mistakenly over-allocate CPU resources to the VMs listed. This can prevent other VMs from turning on or affect the performance of VMs running in the background.

Virtual hard disks

Each VM uses a virtual hard disk (.vhd file). This is where a VM will store files and install programs -- the same way that the host OS uses a physical hard disk. When you are running multiple VMs on a host, remember that a virtual hard disk can only be used by one VM at a time. Therefore, if you plan to run two VMs simultaneously, you will require two virtual hard disks.

Video settings

Since each guest OS runs independent of each other, each one can be configured with different video resolution settings. This will have a slight impact on performance as you switch between VMs running simultaneously. It will take a few seconds for Virtual Server to change to the correct resolution settings as you switch between VMs.

COM and parallel ports

Any devices attached to the host's COM port are accessible under a guest OS. If you plan to run multiple VMs, keep in mind that any device attached to a COM port is only accessible to one VM at a time. If you want to use the device under another guest OS, the other VM must first release the COM port (you can accomplish this by shutting the VM down).

As with COM ports, any devices that are attached to the parallel (LPT1) port is only accessible by one VM at a time. No other guest or host OS will be able to use the device until the VM is shut down.

Floppy drives

Each VM is configured with a single floppy drive. You can configure a VM’s floppy drive to either use a physical floppy disk drive or a virtual floppy disk (.vfd) file.

A virtual floppy disk file can be shared between multiple VMs if the file has been marked as read-only. In the case of a physical floppy disk, this can only be used by one VM at a time.

If you have multiple VMs that are configured to start automatically, consider altering the properties of each one to stagger the start times. By configuring the Seconds to delay automatically starting a VM option, you can prevent the VMs from starting at exactly the same time and thereby reduce the demand placed on the host computer’s physical resources.

Networking your VMs

Multiple VMs can be connected to the same virtual network. Assuming you have already created a new virtual network through the Virtual Server Administration Web site, you can the virtual network adapter for any VM to the virtual network.

You can do this by editing the configuration of a VM via the Administration Web site. Open the Network Adapter properties for the appropriate VM and select the virtual network that you want to connect the virtual network adapter to. There is no limit to the number of VMs that you can connect to a virtual network.

Switching between VMs

Finally, you should know how to actually switch from one running VM to another. Switching between VMs that are running simultaneously is quick to do. You can use the Host key, which by default is the right Alt key. If you have installed the Virtual Machine Additions for each VM, you can switch between them by using your mouse.

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