Difference between revisions of "Creating Xen and KVM RHEL 5 Virtual Machines from the Command-line (virt-install and virsh)"

Revision as of 18:12, 11 May 2016

In the previous chapter we explored the creation of KVM guest operating systems on a Red Hat Enterprise Linux 5 host using the virt-manager graphical tool. Whilst this graphical tool does much to ease the virtual machine creation process, there are some drawbacks to this approach. Perhaps the most significant shortcoming is that the graphical tool does not provide any way to automate creation of virtual machines. In addition, virt-manager can only be used where access to a graphical desktop environment is available. In the case of headless servers, however, this is not always the case.

In this chapter, therefore, we will turn our attention to the creation of KVM and Xen based virtual machines using the virt-install and virsh command-line tools. These tools provide all the capabilities of the virt-manager utility with the added advantage that they can be used within scripts to automate virtual machine creation. In addition, the virsh command allows virtual machines to be created based on a specification contained within a configuration file.

The virt-install tool is supplied to allow new virtual machines to be created by providing a list of command-line options. This chapter assumes that the necessary KVM or Xen tools are installed. For details on these requirements read Installing KVM Virtualization on RHEL 5 or Installing and Configuring RHEL 5 Xen Virtualization.

Running virt-install to build the KVM or Xen Guest System

The virt-install utility must be run as root and accepts a wide range of command-line arguments that are used to provide configuration information related to the virtual machine being created. Some of these command-line options are mandatory (specifically name, ram and disk storage must be provided) while others are optional. A summary of these arguments is outlined in the following table:

An Example RHEL 5 virt-install Command

With reference to the above command-line argument list, we can now look at an example command-line construct using the virt-install tool.

Note that in order to be able to display the virtual machine in a graphical console, the virt-viewer package must be installed on the RHEL host. This can be achieved by executing the following command in a terminal window:

yum install virt-viewer

The following command creates a new KVM or Xen virtual machine configured to run Windows 7 using full virtualization. It creates a new, 10GB disk image, assigns 1024MB of RAM to the virtual machine, configures a CD device for the installation media and uses VNC to display the console:

virt-install --name myWin7 --hvm --ram 1024 --disk path=/tmp/win7.img,size=10 
--network network:default --vnc --os-variant vista --cdrom /dev/hda

Note that the above command line assumes the installation media is in a drive corresponding to device file /dev/hda. This may differ on your system, or may be replaced by a path to an ISO image file residing on a file system.

As the creation process runs, the virt-install command will display status updates of the creation progress:

Starting install...
Creating storage file...                                 | 10.0 GB     00:00
Creating domain...                                       |    0 B     00:00
Domain installation still in progress. Waiting for installation to complete.

Once the guest system has been created, the virt-viewer screen will appear containing the operating system installer loaded from the specified installation media:

Follow the standard installation procedure for the guest operating system.

Starting and Stopping a Virtual Machine from the Command Line

Having created the virtual machine from the command line it stands to reason that you may also need to start it from the command line in the future. This can be achieved using the virsh command line utility, referencing the name assigned to the virtual machine during the creation process. For example:

virsh start myWin7

Similarly, the virtual machine may be sent a shutdown signal as follows:

virsh shutdown myWin7

If the virtual machine fails to respond to the shutdown signal and does not begin a graceful shutdown the virtual machine may be destroyed (with the attendant risks of data loss) using the destroy directive:

virsh destroy myWin7

Creating a Virtual Machine from a Configuration File

The virsh create command can take as an argument the name of a configuration file on which to base the creation of a new virtual machine. The configuration file uses XML format. Arguably the easiest way to create a configuration file is to dump out the configuration of an existing virtual machine and modify it for the new one. This can be achieved using the virsh dumpxml command. The following command outputs the configuration data for a virtual machine domain named WindowsVM to a file named WindowsVM.xml:

virsh dumpxml WindowsVM > WindowsVM2.xml 

Once the file has been generated, load it into an editor to review and change the settings for the new virtual machine:

<domain type='xen'>
  <name>WindowsVM</name>
  <uuid>e33fe24c-0d84-9323-f278-6a949aff0807</uuid>
  <memory>1048576</memory>
  <currentMemory>1048576</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type arch='x86_64' machine='xenfv'>hvm</type>
    <loader>/usr/lib/xen/boot/hvmloader</loader>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <clock offset='localtime'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/lib64/xen/bin/qemu-dm</emulator>
    <disk type='file' device='disk'>
      <driver name='file'/>
      <source file='/var/lib/xen/images/WindowsVM.img'/>
      <target dev='hda' bus='ide'/>
    </disk>
    <disk type='block' device='cdrom'>
      <driver name='phy'/>
      <source dev='/dev/hda'/>
      <target dev='hdc' bus='ide'/>
      <readonly/>
    </disk>
    <interface type='bridge'>
      <mac address='00:16:36:63:a6:de'/>
      <source bridge='virbr0'/>
      <script path='vif-bridge'/>
    </interface>
    <serial type='pty'>
      <target port='0'/>
    </serial>
    <console type='pty'>
      <target port='0'/>
    </console>
    <input type='tablet' bus='usb'/>
    <input type='mouse' bus='ps2'/>
    <graphics type='vnc' port='-1' autoport='yes' keymap='en-us'/>
  </devices>
</domain> 

A variety of settings can be changed. For example the virtualization type, domain name, memory allocation, number of CPUs and the location of the disk image file to name but a few options. At the very least, the domain name and image file must be changed in order to avoid conflict with the virtual machine from which the configuration was taken. Once the file has been modified, the new virtual machine may be created as follows:

virsh create WindowsVM2.xml