Xen 4.1+ with local ZFS storage

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))

1. If you can transfer a maximum of 6GB per day (assuming zero overhead and zero competing traffic) and you are needing to move "15-60 gigs" at a frequency of "once or twice per week," that works out to 15-120 GB per week, or anywhere from 2-17 GB per day. Because it is necessary to plan for peak demand, and 17 GB is far in excess of even your theoretical maximum of 6 GB, it is likely that you have a very serious bandwidth problem. What will it take to upgrade the connection? If upgrading the connection is impossible, please consider the option of mailing physical media on a scheduled basis (e.g. weekly).

2. Assuming that you can get the bandwidth math to make a little bit more sense, rsync is likely to be the best option. Deduplication awareness would be hugely valuable when replicating highly redundant data (e.g. virtual machine images), but it should have little or no benefit when it comes to unique digital content (audio, video, photos)... unless, of course, users are inadvertently storing duplicate copies of identical files.