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About VMs and Templates

Oct 16, 2015

A virtual machine (VM) is a software container (sometimes called a "guest") that runs on a host physical computer and that behaves as if it were a physical computer itself. VMs consist of an operating system plus CPU, memory (RAM) and networking resources, and software applications.

A template is a virtual machine encapsulated into a file, making it possible to rapidly deploy new VMs. Each template contains installation metadata—the setup information needed to create a new VM with a specific guest operating system, and with the optimum storage, CPU, memory and virtual network configuration.

You can create new VMs in XenCenter in a number of different ways:

  • The New VM wizard takes you step by step through the process of creating a new VM from a template or a snapshot, allowing you to configure operating system, CPU, storage, networking and other parameters.
  • You can bypass the New VM wizard and create an "instant VM" based on a custom VM template that specifies all of the required VM configuration parameters. You simply select your preconfigured template in XenCenter then right-click and click Instant VM from template . This mode of unattended VM installation can be useful for deploying large numbers of identical VMs.
  • You can copy (or "clone") an existing VM.
  • You can import a VM that has been previously exported.

XenServer PV drivers - XenServer Tools

VMs in a XenServer environment may be fully virtualized (HVM) or paravirtualized:

  • In HVM (hardware-assisted virtualization or Hardware Virtual Machine) mode, the VM is fully virtualized and can run at near-native processor speeds on virtualization-enabled hardware, without any modification to the guest operating system.
  • In paravirtualized (non-HVM) mode, the guest operating system is tuned and optimized to run in a virtual environment, independent of the underlying processor capabilities. The result is better performance and greater flexibility.

Paravirtualized (PV) drivers are available for Windows and Linux VMs to enhance disk and network performance. These drivers are supplied in the XenServer Tools package and should be installed on all new VMs - see Installing XenServer Tools. XenServer features such as VM migration and historical performance data tracking are only available on VMs that have XenServer Tools installed.

In previous XenServer releases, all supported Linux distributions have operated in PV mode and Windows releases in HVM mode. In XenServer 6.5, the following newly supported Linux distributions will operate in HVM mode:

  • RHEL 7
  • CentOS 7
  • Oracle Linux 7
  • Ubuntu 14.04
Note: For detailed information about supported guest operating systems, refer to the XenServer Virtual Machine User's Guide.

The HVM Linux VMs can take advantage of the x86 virtual container technologies in newer processors for improved performance. Network and Storage access from these VMs will still operate in PV mode, using the drivers built into the kernels. For information about upgrading your existing Linux VMs to versions which now operate in HVM mode, see the Updating Linux Kernels and Guest Utilities section in XenServer Virtual Machine User's Guide.

Using templates

A number of different templates are supplied with XenServer, and these contain all the various configuration settings needed to install a specific guest operating system on a new VM. You can also create your own customized templates configured with the appropriate guest operating system, memory, CPU, storage and network settings, and use them to create new VMs. See XenServer Virtual Machine User's Guide for a list of the templates/operating systems supported at this release, and for detailed information about the different install mechanisms on Windows and Linux.

You can view the XenServer templates supplied with the product and any custom templates that you create in the Resources pane.

  • XenServer template icon XenServer template
  • Custom template icon Custom template

You can control whether or not to display the XenServer and Custom templates in the Resources pane:

  • In the XenCenter Navigation pane, click Infrastructure.

    This displays a tree view of your managed resources in the Resources pane.

  • To display standard XenServer VM templates: on the View menu, click XenServer Templates; to hide XenServer templates, click again to remove the check mark.
  • To show custom VM templates: on the View menu, click Custom Templates; to hide custom templates, click again to remove the check mark.