Linux – Which Linux-Virtualization to choose for Server

Your questions is a little unclear as to what kind of stats you are hoping to collect from the dom0, but I think what you are looking for is an understanding of how your existing hardware resources are allocated.

We're having great success using libvirt's Python bindings to get this information. Here's a Python script snippet that illustrates the idea:

#!/usr/bin/env python

import sys
import libvirt

def main(options,args):
    hypervisors = sys.argv[1:]

    print "%16s%18s%18s%18s" % ("dom0 IP", "Free Memory (MB)", "Disk Used (GB)", "Disk Free (GB)")
    for ip in hypervisors:
        # this assumes "remote" connection to libvirtd using TCP
        url = "xen+tcp://%s" % (ip)

        conn = libvirt.open(url)
        # you may want to do more error handling here
        if conn == None:
            continue

        mem = conn.getFreeMemory() / 1048576 #convert bytes -> megabytes
        pool = conn.storagePoolLookupByName('vol0')
        # a refresh() is necessary because libvirtd's internal information isn't
        # always in sync with the host.
        pool.refresh(0)
        disk_info = pool.info()
        disk_used = disk_info[2] / 1073741824 #convert bytes -> gigabytes
        disk_free = disk_info[3] / 1073741824 #convert bytes -> gigabytes
        print "%16s%18d%18s%18d" % (ip, mem, disk_used, disk_free)

if __name__ == '__main__':
    sys.exit(main(options,args))

Performance loss - Yes, technically there is one. Is it something you or your users will notice? Unless it's some crazy high-end workload, or you horribly over-provision VM's* (or are trying to squeeze 5 VM's with a "normal" RAM allocation onto an old, existing server, I seriously doubt it. Remember to actually check your RAM usage - if you're splitting everything up, you don't need, say, 512megs for an NTP server that defaults to runlevel 3. (Splitting off JUST an NTP server is excessive, it was merely an example.)

Work - This is true. If it's only going from, say, one to three servers, probably not that big a deal - do your change, copy/paste your commands from one terminal session to the other. Past that though, you want some kind of management tool, I'm currently looking at Puppet.

*Memory Limits - Depends on the virt. solution you use. Some environments, like ESX/vSphere allow you to allocate more RAM to VM's than physically is available. If you pay for the feature, ESX let's you set up resource pools, and will automatically adjust resources as needed, with the ability to set priorities. Like everything, you have to know how it works and the tradeoffs in a particular environment.