[clug] Virt-Manager & snapshots Vs Xen

Bob Edwards Robert.Edwards at anu.edu.au
Tue Sep 30 17:29:49 MDT 2014

On 30/09/14 23:02, George at Clug wrote:
> I was using KVM with Virt-Manager under Centos 7 and Debian Wheezy and
> when I went to take a snapshot of a 6 TB server before applying
> updates, and found that there was no snapshot support. For me
> snapshots are a base requirement for any virtual machine. For example,
> before applying any updates to a virtual server, take snapshot,
> install updates, test running software to see nothing as broken,
> delete snapshot while consoldiating image, or if the changes cause
> issues, roll back the image to the snapshot and live without the
> updates until you can apply them without causing issues.
> Searching on the Internet I discovered that Virt-Manager >= version
> 1.0 can take snapshots if the image is qcow2.
> http://blog.wikichoon.com/2014/03/snapshot-support-in-virt-manager.html
> The biggest feature we added in virt-manager 1.0 is VM snapshot
> support. ... Only works with qcow2 disk images. ... While the overlay
> files have to be qcow2, these snapshots will work with any base disk
> image. They can also be performed with very little VM downtime, at
> least under a second.
> When checking both Debian Jessie and CentOS 7, I discovered that
> Virt-Manager was < version 1.0, so using standard packages was not
> going to be a useful solution. I will be pleased with these OSs
> upgrade to a virt-manager that can support snapshots,but it could be a
> long while away.
> Going to the Virt-Manager home page, I downloaded the latest verison
> of the source code, "virt-manager 1.1.0 Sunday September 07, 2014",
> http://virt-manager.org/download/
> "To build an RPM, you can run:    python setup.py rpm"
> When I attempted to compile an RPM for CentOS 7, the compile failed
> with "Requires: libosinfo >= 0.2.10".
> ftp://ftp.isu.edu.tw/pub/Linux/Fedora/linux/releases/test/21-Alpha/Workstation/x86_64/os/Packages/l/libosinfo-0.2.11-1.fc21.x86_64.rpm
> http://pkgs.org/fedora-21/fedora-x86_64/libosinfo-0.2.11-2.fc21.x86_64.rpm.html
> http://dl.fedoraproject.org/pub/fedora/linux/development/21/x86_64/os/Packages/l/libosinfo-0.2.11-2.fc21.x86_64.rpm
> The above provided an RPM for libosinfo which allowed Virt-Manager to
> compile.
> Now I if I create a qcow2 based virtual machine, I can take snapshots,
> though I have not yet had time to test the stability or effectiveness
> of these snapshots.
> I would be interested in peoples comments on the above.
> I originaly dismissed Xen as I believed it was tied to a commercial
> company, and not completely open to the Open Source community.  If I
> am wrong please correct me. Also let me know what type of management
> interface Xen can use.
> http://en.wikipedia.org/wiki/Xen
> The Xen community develops and maintains Xen as free and open-source
> software, subject to the requirements of the GNU General Public
> License (GPL), version 2.

As I understand it, there is nothing sinister about Xen. What is
significant is that the Xen kernel module never made it into the
Torvalds (official) kernel, whereas KVM did. So, KVM now exists
in all Linux kernels, whereas Xen needs external kernel modules to
be loaded etc. for it to work.

As for CoW, I think several work-arounds are:
  - use btrfs - has snapshots etc. already
  - use LVM - similarly has snapshots

(note, I haven't actually done this with KVM yet - still getting

VirtualBox is also GPL and does have snapshots and a decent management
interface etc. Again, has its own kernel module to load, but does have
the (possible) advantage that it runs on other platforms (Mac OS X and
that other one). Probably "heavier-weight" than Xen or KVM in terms of
host resources.

Bob Edwards.

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