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
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