How to change the XML-description of a virtual machine

If you need to delete an XML document that is passed to libvirt while creating or editing a virtual machine, you can use our handler that is processed on the last step before the XML is sent to libvirt. You can add a plug-in and edit the XML description according to your needs.

The handler name is editxml

Input parameters:

vmi - OS template name (for version 5.22 and later)
hostnode - ID of the cluster node where the virtual machine is located

XML for libvirt is located in the /doc/outxml tag

VMmanager also read the XML from the /doc/outxml tag

Handler example

Task

Add the pm (Power management) tags into the XML

...
<pm>
  <suspend-to-disk enabled='yes'/>
  <suspend-to-mem enabled='yes'/>  
</pm>
...

For OS template "SlitazOs" find all virtual disks with the "block" blcok type and change caching into "writeback"

Solution

Create a description file for your handler and put it into the /usr/local/mgr5/etc/xml directory

The /usr/local/mgr5/etc/xml/vmmgr_mod_pm.xml file:

<mgrdata>
  <handler name="editxml_pm.py" type="xml">
  <event name="editxml" after="yes"/>
</handler>
</mgrdata>

We will also need the editxml_pm.py script, which should locate in /usr/local/mgr5/addon

The /usr/local/mgr5/addon/editxml_pm.py file

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import sys
import xml.etree.ElementTree as ET

# Contents of  VMmanager session in the form of an XML-file
sesXmlContent = sys.stdin.read()
ses = ET.fromstring(sesXmlContent)
# Retrieve an OS template name 
vmi = ses.find("./vmi").text
# Retrieve the cluster node ID
hostNodeId = ses.find("./hostnode").text
# Original xml of the virtual machine
domainXmlString = ses.find("./outxml").text
domainXml = ET.fromstring(domainXmlString)
# Retrieve the virtual machine name
domainName = domainXml.find("./name").text
#Edit the xml only for SlitazOs
if vmi == "SlitazOs":
    # Add the tag <pm> (Power Management)
    pm = ET.SubElement(domainXml, "pm")
    suspendToDisk = ET.SubElement(pm, "suspend-to-disk", {"enabled": "yes"})
    suspendToMem = ET.SubElement(pm, "suspend-to-mem", {"enabled": "yes"})
    # Search for all connected disks
    disks = domainXml.findall("./devices/disk")
    for diskNode in disks:
        # Get a virtual disk type  (file, network, block, volume)
        diskType = diskNode.get("type")
        # Edit only for  "block"
        if diskType == "block":
            # Set the caching type  "writeback"
            driver = diskNode.find("driver")
            driver.set("cache", "writeback")

# Generate the output XML
newDomContent = ET.tostring(domainXml, "UTF-8")
ses.find("./outxml").text = newDomContent
# XML output
sys.stdout.write(ET.tostring(ses, "UTF-8"))

For the /usr/local/mgr5/addon/editxml_pm.py file you need to set start permissions:

chmod +x /usr/local/mgr5/addon/editxml_pm.py

Restart VMmanager:

killall core

You will see the following record in vmmgr.log

Dec  9 06:48:45 [15118:1] action EXTINFO Register event 'editxml_pm.py' for action 'editxml'

This add-on is ready to use. Now, when you edit a virtual machine, our add-on will process and modify its XML.

In order to regenerate XML of the virtual machine without editing it in the control panel, execute the following request:

/usr/local/mgr5/sbin/mgrctl -m vmmgr vm.redefine elid=<VM_ID>

The article was last updated on 11.27.2018. The article was prepared by technical writers of ISPsystem.