Installing a CentOS KVM Guest OS from the Command-line (virt-install)

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Creating a CentOS KVM Virtual MachineManaging and Monitoring CentOS based KVM Guest Systems


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In the previous chapter we explored the creation of KVM guest operating systems on a CentOS 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 guest operating system using the virt-install command-line tool. This tool provides all the capabilities of the virt-manager utility with the added advantage that it can be used within scripts to automate virtual machine creation.

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 tools are installed. For details on these requirements read Installing KVM on CentOS and Configuring a Network Bridge.

Running virt-install to Build the KVM 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:

Argument

Description

-h, --help Show the help message and exit
--connect=CONNECT Connect to a non-default hypervisor.
-n NAME, --name=NAME Name of the new guest virtual machine instance. This must be unique amongst all guests known to the hypervisor on the connection, including those not currently active. To re-define an existing guest, use the virsh(1) tool to shut it down (’virsh shutdown’) & delete (’virsh undefine’) it prior to running "virt-install".
-r MEMORY, --ram=MEMORY Memory to allocate for guest instance in megabytes. If the hypervisor does not have enough free memory, it is usual for it to automatically take memory away from the host operating system to satisfy this allocation.
--arch=ARCH Request a non-native CPU architecture for the guest virtual machine. The option is only currently available with QEMU guests, and will not enable use of acceleration. If omitted, the host CPU architecture will be used in the guest.
-u UUID, --uuid=UUID UUID for the guest; if none is given a random UUID will be generated. If you specify UUID, you should use a 32-digit hexadecimal number. UUID are intended to be unique across the entire data center, and indeed world. Bear this in mind if manually specifying a UUID
--vcpus=VCPUS Number of virtual cpus to configure for the guest. Not all hypervisors support SMP guests, in which case this argument will be silently ignored
--check-cpu Check that the number virtual cpus requested does not exceed physical CPUs and warn if they do.
--cpuset=CPUSET Set which physical cpus the guest can use. "CPUSET" is a comma separated list of numbers, which can also be specified in ranges. If the value ’auto’ is passed, virt-install attempts to automatically determine an optimal cpu pinning using NUMA data, if available.
--os-type=OS_TYPE Optimize the guest configuration for a type of operating system (ex. ’linux’, ’windows’). This will attempt to pick the most suitable ACPI & APIC settings, optimally supported mouse drivers, virtio, and generally accommodate other operating system quirks. See "--os-variant" for valid options. For a full list of valid options refer to the man page (man virt-install).
--os-variant=OS_VARIANT Further optimize the guest configuration for a specific operating system variant (ex. ’fedora8’, ’winxp’). This parameter is optional, and does not require an "--os-type" to be specified. For a full list of valid options refer to the man page (man virt-install).
--host-device=HOSTDEV Attach a physical host device to the guest. HOSTDEV is a node device name as used by libvirt (as shown by ’virsh nodedev-list’).
--sound Attach a virtual audio device to the guest. (Full virtualization only).
--noacpi Override the OS type / variant to disables the ACPI setting for fully virtualized guest. (Full virtualization only).
-v, --hvm Request the use of full virtualization, if both para & full virtualization are available on the host. This parameter may not be available if connecting to a Xen hypervisor on a machine without hardware virtualization support. This parameter is implied if connecting to a QEMU based hypervisor.
-p, --paravirt This guest should be a paravirtualized guest. If the host supports both para & full virtualization, and neither this parameter nor the "--hvm" are specified, this will be assumed.
--accelerate When installing a QEMU guest, make use of the KVM or KQEMU kernel acceleration capabilities if available. Use of this option is recommended unless a guest OS is known to be incompatible with the accelerators. The KVM accelerator is preferred over KQEMU if both are available.
-c CDROM, --cdrom=CDROM File or device use as a virtual CD-ROM device for fully virtualized guests. It can be path to an ISO image, or to a CDROM device. It can also be a URL from which to fetch/access a minimal boot ISO image. The URLs take the same format as described for the "--location" argument. If a cdrom has been specified via the "--disk" option, and neither "--cdrom" nor any other install option is specified, the "--disk" cdrom is used as the install media.
-l LOCATION, --location=LOCATION Installation source for guest virtual machine kernel+initrd pair. The "LOCATION" can take one of the following forms:
  • DIRECTORY - Path to a local directory containing an installable distribution image
  • nfs:host:/path or nfs://host/path - An NFS server location containing an installable distribution image
  • http://host/path - An HTTP server location containing an installable distribution image
  • ftp://host/path - An FTP server location containing an installable distribution image
--pxe Use the PXE boot protocol to load the initial ramdisk and kernel for starting the guest installation process.
--import Skip the OS installation process, and build a guest around an existing disk image. The device used for booting is the first device specified via "--disk" or "--file".
--livecd Specify that the installation media is a live CD and thus the guest needs to be configured to boot off the CDROM device permanently. It may be desirable to also use the "--nodisks" flag in combination.
-x EXTRA, --extra-args=EXTRA Additional kernel command line arguments to pass to the installer when performing a guest install from "--location".
--disk=DISKOPTS Specifies media to use as storage for the guest, with various options.
--disk opt1=val1,opt2=val2,... To specify media, one of the following options is required:
  • path - A path to some storage media to use, existing or not. Existing media can be a file or block device. If installing on a remote host, the existing media must be shared as a libvirt storage volume. Specifying a non-existent path implies attempting to create the new storage, and will require specifyng a ’size’ value. If the base directory of the path is a libvirt storage pool on the host, the new storage will be created as a libvirt storage volume. For remote hosts, the base directory is required to be a storage pool if using this method.
  • pool - An existing libvirt storage pool name to create new storage on. Requires specifying a ’size’ value.
  • vol - An existing libvirt storage volume to use. This is specified as ’poolname/volname’.
  • device - Disk device type. Value can be ’cdrom’, ’disk’, or ’floppy’. Default is ’disk’. If a ’cdrom’ is specified, and no install method is chosen, the cdrom is used as the install media.
  • bus - Disk bus type. Value can be ’ide’, ’scsi’, ’usb’, ’virtio’ or ’xen’. The default is hypervisor dependent since not all hypervisors support all bus types.
  • perms - Disk permissions. Value can be ’rw’ (Read/Write), ’ro’ (Readonly), or ’sh’ (Shared Read/Write). Default is ’rw’
  • size - size (in GB) to use if creating new storage
  • sparse - whether to skip fully allocating newly created storage. Value is ’true’ or ’false’. Default is ’true’ (do not fully allocate). The initial time taken to fully-allocate the guest virtual disk (spare=false) will be usually by balanced by faster install times inside the guest. Thus use of this option is recommended to ensure consistently high performance and to avoid I/O errors in the guest should the host filesystem fill up.
  • cache - The cache mode to be used. The host pagecache provides cache memory. The cache value can be ’none’, ’writethrough’, or ’writeback’. ’writethrough’ provides read caching. ’writeback’ provides read and write caching. See the examples section for some uses. This option deprecates "--file", "--file-size", and "--nonsparse".
-f DISKFILE, --file=DISKFILE Path to the file, disk partition, or logical volume to use as the backing store for the guest’s virtual disk. This option is deprecated in favor of "--disk".
-s DISKSIZE, --file-size=DISKSIZE Size of the file to create for the guest virtual disk. This is deprecated in favor of "--disk".
--nonsparse Fully allocate the storage when creating. This is deprecated in favort of "--disk"
--nodisks Request a virtual machine without any local disk storage, typically used for running ’Live CD’ images or installing to network storage (iSCSI or NFS root).
-w NETWORK, --network=NETWORK Connect the guest to the host network. The value for "NETWORK" can take one of 3 formats:
  • bridge:BRIDGE - Connect to a bridge device in the host called "BRIDGE". Use this option if the host has static networking config & the guest requires full outbound and inbound connectivity to/from the LAN. Also use this if live migration will be used with this guest.
  • network:NAME - Connect to a virtual network in the host called "NAME". Virtual networks can be listed, created, deleted using the "virsh" command line tool. In an unmodified install of "libvirt" there is usually a virtual network with a name of "default". Use a virtual network if the host has dynamic networking (eg NetworkManager), or using wireless. The guest will be NATed to the LAN by whichever connection is active.
  • user - Connect to the LAN using SLIRP. Only use this if running a QEMU guest as an unprivileged user. This provides a very limited form of NAT.
  • If this option is omitted a single NIC will be created in the guest. If there is a bridge device in the host with a physical interface enslaved, that will be used for connectivity. Failing that, the virtual network called "default" will be used. This option can be specified multiple times to setup more than one NIC.
-b BRIDGE, --bridge=BRIDGE Bridge device to connect the guest NIC to. This parameter is deprecated in favour of the "--network" parameter.
-m MAC, --mac=MAC Fixed MAC address for the guest; If this parameter is omitted, or the value "RANDOM" is specified a suitable address will be randomly generated. For Xen virtual machines it is required that the first 3 pairs in the MAC address be the sequence ’00:16:3e’, while for QEMU or KVM virtual machines it must be ’54:52:00’.
--nonetworks Request a virtual machine without any network interfaces.
--vnc Setup a virtual console in the guest and export it as a VNC server in the host. Unless the "--vncport" parameter is also provided, the VNC server will run on the first free port number at 5900 or above. The actual VNC display allocated can be obtained using the "vncdisplay" command to "virsh" (or virt-viewer(1) can be used which handles this detail for the use).
--vncport=VNCPORT Request a permanent, statically assigned port number for the guest VNC console. Use of this option is discouraged as other guests may automatically choose to run on this port causing a clash.
--sdl Setup a virtual console in the guest and display an SDL window in the host to render the output. If the SDL window is closed the guest may be unconditionally terminated.
--nographics No graphical console will be allocated for the guest. Fully virtualized guests (Xen FV or QEmu/KVM) will need to have a text console configured on the first serial port in the guest (this can be done via the --extra-args option). Xen PV will set this up automatically. The command ’virsh console NAME’ can be used to connect to the serial device.
--noautoconsole Don’t automatically try to connect to the guest console. The default behaviour is to launch a VNC client to display the graphical console, or to run the "virsh" "console" command to display the text console. Use of this parameter will disable this behaviour.
-k KEYMAP, --keymap=KEYMAP Request that the virtual VNC console be configured to run with a non- English keyboard layout.
-d, --debug Print debugging information to the terminal when running the install process. The debugging information is also stored in "$HOME/.virtinst/virt-install.log" even if this parameter is omitted.
--noreboot Prevent the domain from automatically rebooting after the install has completed.
--wait=WAIT Amount of time to wait (in minutes) for a VM to complete its install. Without this option, virt-install will wait for the console to close (not neccessarily indicating the guest has shutdown), or in the case of --noautoconsole, simply kick off the install and exit. Any negative value will make virt-install wait indefinitely, a value of 0 triggers the same results as noautoconsole. If the time limit is succeeded, virt-install simply exits, leaving the virtual machine in its current state.
--force Prevent interactive prompts. If the intended prompt was a yes/no prompt, always say yes. For any other prompts, the application will exit.
--prompt Specifically enable prompting. Default prompting is off (as of virtinst 0.400.0)

An Example CentOS 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.

The following command creates a new KVM virtual machine configured to run Windows 7 using full virtualization. It creates a new, 10GB disk image, assigns 512MB 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 512 --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...                                 | 6.0 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:


CentOS KVM guest running after command line installation using virt-install


Follow the standard installation procedure for the guest operating system.


Starting the 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


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Kindle/ePub/PDF edition contains 39 chapters and over 240 pages.

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PreviousTable of ContentsNext
Creating a CentOS KVM Virtual MachineManaging and Monitoring CentOS based KVM Guest Systems