[libvirt-users] Virsh snapshots

[Kashyap Chamarthy]

On 12/02/2013 05:04 PM, Juraj Melo wrote:
> Hello,
> 
> I am working on my PhD thesis and it would be really helpfull if someone
> could advise me, whether can Virsh create snapshots of VMs using
> copy-on-write.

You might want to describe a little bit more what you want to accomplish, because, libvirt
already has some advanced snapshotting mechanisms.

.
Here's some info about libvirt snapshots:


A brief introduction of different *flavours* of snapshots (& terminology):

 1) Internal Snapshots, these can be:

     1.1. Internal 'disk snapshot' -- {live/offline}
     1.2. Internal 'system checkpoint' snapshot -- {live}

  With Internal Snapshots, a single qcow2 file holds everything: original & its delta.
  The original file /must/ be a QCOW2 file.

  NOTE: 'system checkpoint' meaning -- it captures VM state and disk state. Where
  VM state indicates: it captures memory and device state (but not "disk" state).


 2) External Snapshots, these can be:

     2.1. External 'disk snapshot' -- {live/offline}. i.e: snapshot of the
          disk is saved in one file, and the delta since the snapshot is
          tracked in a new qcow2 file.

     2.2. External 'system checkpoint' -- {live}. i.e: the guest's
          disk-state will be saved in one file, its RAM & device-state
          will be saved in another new file.


If you want to read more about overlays with examples (and reducing snapshot chains,
here's more information than ever :)  --
http://kashyapc.fedorapeople.org/virt/lc-2012/snapshots-handout.html



Getting started
===============

If you just want to get started w/ snapshotting guests, here's how I do.
I use external live snapshots,  -- fastest (more on it further below):


(1) List the block device associated with a guest.

    $ virsh domblklist f19-base
    Target     Source
    ---------------------------------------------
    vda        /export/vmimages/f19-base.qcow2

    $


(2) Create external disk-only snapshot (while the guest is *running*).

    $ virsh snapshot-create-as --domain f19-base snap1 snap1-desc \
      --disk-only \
      --diskspec vda,snapshot=external,file=/export/vmimages/overlay1-of-f19-base.qcow2 \
      --atomic
    Domain snapshot snap1 created
    $

    NOTE-1: Once the above command is issued, the original disk-image
    of f19-base will become the backing_file & a new overlay
    image is created to track the new changes. Here on, libvirt
    will use this overlay for further write operations(while
    using the original image as a read-only backing_file).

    NOTE-2: You can also use a simpler command-line like below,
    but, you cannot specify the name & location of your overlay-file:

      $ virsh snapshot-create-as f19-base snap1 "snap1-desc" \
        --disk-only --atomic


(3) Now, list the block device associated(use command from step-1, above)
    with the guest, to ensure it reflects the new overlay image as
    the current block device in use.

    $ virsh domblklist f19-base
    Target     Source
    ----------------------------------------------------
    vda        /export/vmimages/overlay1-of-f19-base.qcow2
    $


Here's some more examples:
http://kashyapc.fedorapeople.org/virt/lc-2012/snapshots-illustration.txt


Some URLs
=========

1.

http://wiki.libvirt.org/page/I_created_an_external_snapshot,_but_libvirt_won%27t_let_me_delete_or_revert_to_it


2. Notes on different kinds of snapshots --
http://kashyapc.fedorapeople.org/virt/lc-2012/snapshots-handout.html





-- 
/kashyap