Wednesday, February 2, 2011

Manage Xen by libvirt tools

libvirt is an open source API, daemon and management tool for managing platform virtualization
It is much easier to use than the xen native tools for VM creation, network management and storage management
- Standard, easy and neat commands to manage VM creation, network management and storage management.
- Support all well known hypervisors (Linux KVM, Xen, VMware ESX, OpenVZ..), so the knowledge is transferable.
- Remote management with TLS encryption and Kerberos authentication.
- API bindings for multiple languages: Python,Perl,Ruby, Java, OCaml , C#, and PHP
- Operation isolation: Stopping libvirt (version> 0.6.0) daemon won't affect VM
- Libvirt couldn’t keep up with the development of the underlying hypervisor, so it might not be able understand new features in hypervisor.
- An additional layer of management introduces availability and security concerns. Although stopping libvirt daemon won’t affect VM, but if libvrit daemon fails upon hypervisor reboot. The network bridge managed by libvirt won’t be created. But it can be quickly remedied by simple command:
$brctl addbr br-name; ifconfig br-name IP up
Where does libvirt save the VM configuration file?
It depends on the hypervisor. For Xen, libvirt use Xen API to save it to xenstore(/var/lib/xenstored). Because xenstore is Xen component, that is why VM native tools can start VM without libvirt daemon.
The following stript can be used to examine the VM configuration.
function dumpkey() {
local param=${1}
local key
local result
result=$(xenstore-list ${param})
if [ "${result}" != "" ] ; then
for key in ${result} ; do dumpkey ${param}/${key} ; done
echo -n ${param}'='
xenstore-read ${param}
for key in /vm /local/domain /tool ; do dumpkey ${key} ; done
Install libvirt on Debian
$apt-get install libvirt-bin virtinst
Enable xend-unix-server in xend to talk to libvirt
$ grep xend-unix-server /etc/xen/xend-config.sxp
(xend-unix-server yes)

$/etc/init.d/xend restart
Define new network bridge
root@xen4:/etc/xen# cat /tmp/net.xml
<bridge name="virbr2" />
<ip address="" netmask="">
Type “virsh” to enter an virsh interactive prompt
virsh # net-define /tmp/net.xml
Network private defined from /tmp/net.xml
virsh # net-autostart  private
Network private marked as autostarted
virsh # net-start  private
Network private started
virsh # net-list
Name                 State      Autostart
private              active     yes
root@xen4:/# ifconfig  virbr2
virbr2    Link encap:Ethernet  HWaddr de:49:4e:43:c5:5d
inet addr:  Bcast:  Mask:
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)
Install Centos para-virtualization guest.
#Prepare sparse disk file with qemu-img tool
$qemu-img create -f raw /data/pv2.raw 2G
Para virtualization guest can’t use cdrom as install source, In this example, I mount the ISO file to a web server directory.
$virt-install \
--paravirt \
--name pv2 \
--ram 256 \
--disk path=/data/pv2.raw,size=2,format=raw \
--os-type=linux --os-variant=rhel5.4 \
--nographics \
--network network=private \
After the VM has been created, you can use xen native tool /usr/sbin/xm or libvirt virsh command to start/stop VM. But any configuration change require the virsh edit commands (edit, net-edit, pool-edit vol-edit, iface-edit)

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