libvirt users - Oct 2013 - Snapshots: Where they are stored and how to use them?

Firstly I'd like to learn where a snapshot is stored once it has been
made.  I understand that the principle is that when changes occur in
the VM, the difference is written to the snapshot, thus recording the
differences between the date/time of the snapshot and the current
value of the VM.  Is that a correct assessment of the process?

If that is indeed so, can I take regular snapshots and back then up
onto external storage to provide a sort of system restore point at
certain times in the past?

I have scanned the server hard disks for the snapshot images after
running "sudo virsh snapshot-create Windows /etc/libvirt/snapshot.xml"
and the system reports a snapshot was taken, yet I cannot find any
file that contains the snapshot.

$ sudo virsh snapshot-list Windows
 Name                 Creation Time             State
------------------------------------------------------------
 Windows              2013-10-02 12:41:06 +0200 running

What will happen to the snapshot if the host server is restarted?

Some insight would be greatly appreciated.

regards

Roland

On 10/02/2013 09:56 AM, Roland Giesler wrote:
> Firstly I'd like to learn where a snapshot is stored once it has been
> made.
That depends - did you create an external or an internal snapshot?
'virsh snapshot-info Windows Windows' will help.
>  I understand that the principle is that when changes occur in
> the VM, the difference is written to the snapshot, thus recording the
> differences between the date/time of the snapshot and the current
> value of the VM.  Is that a correct assessment of the process?
Sort of.  The differences aren't recorded in the snapshot, so much as it
records the contents at the point in time it was taken, and running the
VM now tracks the differences compared to when the snapshot was taken.
That is, the snapshot is not being modified, but the current state.
> 
> If that is indeed so, can I take regular snapshots and back then up
> onto external storage to provide a sort of system restore point at
> certain times in the past?
Yes, that is one use for snapshots (done better with external snapshots,
because qemu 1.6 lacks the ability to let you view the contents of
internal snapshots while the domain is running - patches have been
proposed for qemu 1.7 to start addressing that, but then libvirt would
also have to be taught to take advantage of the new qemu feature).  But
be aware that external snapshots are still not fully implemented in
libvirt - while you can create them, we do not yet have decent support
in place for libvirt-only revert or delete of a snapshot.  See this wiki
page for more than you ever wanted to know about management of external
snapshots (and update it, if you'd like):

http://wiki.libvirt.org/page/I_created_an_external_snapshot%2C_but_libvirt_won%27t_let_me_delete_or_revert_to_it
> 
> I have scanned the server hard disks for the snapshot images after
> running "sudo virsh snapshot-create Windows
/etc/libvirt/snapshot.xml"
> and the system reports a snapshot was taken, yet I cannot find any
> file that contains the snapshot.
> 
> $ sudo virsh snapshot-list Windows
>  Name                 Creation Time             State
> ------------------------------------------------------------
>  Windows              2013-10-02 12:41:06 +0200 running
Then you probably took an internal snapshot, which means that the very
qcow2 file that is hosting the disk image is also holding the snapshot.
 'qemu-img info /path/to/qcow2' would confirm this.
> 
> What will happen to the snapshot if the host server is restarted?
It will still be there, ready for you if you ever want to revert to it.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

On 10/02/2013 09:26 PM, Roland Giesler wrote:
> Firstly I'd like to learn where a snapshot is stored once it has been
> made.  
If you've created external snapshots, then all the files should be under

  /var/lib/libvirt/images/


Here's just a quick test I did on my laptop:

Before snapshot:
===============
# List the current block device of your guest
$ virsh domblklist f19oz
Target     Source
----------------------------------------------
vda        /var/lib/libvirt/images/f19oz.dsk


Create an external disk-only snapshot:
=====================================
Note, the guest was running when I did the below:

$ virsh snapshot-create-as f19oz snap1 "snap1-desc" --disk-only
--atomic
Domain snapshot snap1 created

After snapshot:
==============
# Notice how the current disk image is the new one ending with .snap1

$ virsh domblklist f19oz
Target     Source
------------------------------------------------
vda        /var/lib/libvirt/images/f19oz.snap1


Optionally:View the entire backing chain of your images:

$ qemu-img info --backing-chain /var/lib/libvirt/images/f19oz.snap1
image: /var/lib/libvirt/images/f19oz.snap1
file format: qcow2
virtual size: 3.0G (3221225472 bytes)
disk size: 196K
cluster_size: 65536
backing file: /var/lib/libvirt/images/f19oz.dsk
backing file format: raw

image: /var/lib/libvirt/images/f19oz.dsk
file format: raw
virtual size: 3.0G (3221225472 bytes)
disk size: 834M

> I understand that the principle is that when changes occur in
> the VM, the difference is written to the snapshot, thus recording the
> differences between the date/time of the snapshot and the current
> value of the VM.  Is that a correct assessment of the process?
True. There are several different flavours of snapshots. These are discussed
elsewhere,
writing it here for convenience:


 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 that is indeed so, can I take regular snapshots and back then up
> onto external storage to provide a sort of system restore point at
> certain times in the past?
I don't know what you mean by "regular snapshots" here.

However, an example of live backup using libvirt's blockcopy --
http://kashyapc.fedorapeople.org/virt/lc-2012/live-backup-with-blockcopy.txt
> 
> I have scanned the server hard disks for the snapshot images after
> running "sudo virsh snapshot-create Windows
/etc/libvirt/snapshot.xml"
I think you created an 'internal' snapshot (defined above) here.
Meaning, your Original
and the delta are stored in the single file. So you'll not find snapshot
images files
lying around.

Examining the content of your /etc/libvirt/snapshot.xml would be more useful.

> and the system reports a snapshot was taken, yet I cannot find any
> file that contains the snapshot.
> 
> $ sudo virsh snapshot-list Windows
>  Name                 Creation Time             State
> ------------------------------------------------------------
>  Windows              2013-10-02 12:41:06 +0200 running
> 
> What will happen to the snapshot if the host server is restarted?
The snapshot is still preserved.
> 
> Some insight would be greatly appreciated.

PS: This presentation may give you more idea on different snapshots available --
http://kashyapc.fedorapeople.org/virt/lc-2012/lceu-2012-virt-snapshots-kashyap-chamarthy.pdf

And, some examples listed here (slightly old, but should still be valid) --
http://kashyapc.fedorapeople.org/virt/lc-2012/snapshots-illustration.txt

Hope that helps.

-- 
/kashyap