libvirt users - May 2014 - Re: Live snapshots of a single block device

Hello,

I am working on a script to automatically create live snapshots of running VMs
using qemu-kvm 1.4.0 and libvirt 1.0.2. If a VM has multiple disks, I'd like
to back them up individually with separate calls to snapshot-create-as, so I can
more easily manage the disk images. The code I have now is essentially as
follows:
virsh snapshot-create-as --domain "vmname" --name
"snapshotname" --description "snapshot description"
--disk-only --atomic --no-metadata --diskspec "vda"

I run this command for each block device (e.g vda, vdb, etc). However, after
running the above command (to only backup vda), I see that a snapshot has also
been created for vdb. I tried running it with --print-xml, which gives this
output:
<domainsnapshot>
  <name>05-22-14_17-09-25</name>
  <description>05-22-14_17-09-25</description>
  <disks>
    <disk name='vda'/>
  </disks>
</domainsnapshot>

What am I doing wrong - how can I tell snapshot-create-as to create an external
snapshot for a specific block device only (not all block devices)?

Also, while looking at the manpage, does the --live option do anything different
if used with the above command?

Thanks,

Andrew Martin

On 05/22/2014 04:31 PM, Andrew Martin wrote:
> Hello,
> 
[Can you convince your mailer to wrap long lines?]
> I am working on a script to automatically create live snapshots of
> running VMs using qemu-kvm 1.4.0 and libvirt 1.0.2. If a VM has
Any reason you aren't upgrading to newer versions?  Although these seem
sufficient for what you are trying.
> multiple disks, I'd like to back them up individually with separate
> calls to snapshot-create-as, so I can more easily manage the disk
> images. The code I have now is essentially as follows:
> virsh snapshot-create-as --domain "vmname" --name
"snapshotname" --description "snapshot description"
--disk-only --atomic --no-metadata --diskspec "vda"
> 
> I run this command for each block device (e.g vda, vdb, etc). However,
after running the above command (to only backup vda), I see that a snapshot has
also been created for vdb. I tried running it with --print-xml, which gives this
output:
> <domainsnapshot>
>   <name>05-22-14_17-09-25</name>
>   <description>05-22-14_17-09-25</description>
>   <disks>
>     <disk name='vda'/>
>   </disks>
The problem you are facing is that the snapshot code MUST have an action
for every disk; and if you don't specify the action directly in your
<domainsnapshot> xml, then the action chosen is inherited from the
<domain> xml (if you used <disk snapshot='no'>) or
ultimately from the
type of snapshot you are creating (here, since you didn't use <disk
snapshot='no'> in the domain xml, and didn't specify vdb in the
<domainsnapshot>, libvirt assumes from --disk-only that you want vdb to
have an external snapshot taken).
> </domainsnapshot>
> 
> What am I doing wrong - how can I tell snapshot-create-as to create an
external snapshot for a specific block device only (not all block devices)?
Use: --diskspec vda --diskspec vdb,snapshot=no

which produces this subset of xml:

  <disks>
    <disk name='vda'/>
    <disk name='vdb' snapshot='no'/>
  </disks>
> 
> Also, while looking at the manpage, does the --live option do anything
different if used with the above command?
--live only makes sense when mixed with memory snapshots (with
--memspec); but as you are doing a --disk-only snapshot, it doesn't help
(I'm not sure if it will error out as mutually exclusive or just be
silently ignored, without reading the code).

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

----- Original Message -----
> From: "Eric Blake" <eblake@redhat.com> To: "Andrew
Martin"
> <amartin@xes-inc.com>, libvirt-users@redhat.com Sent: Thursday, May
22, 2014
> 5:44:54 PM Subject: Re: [libvirt-users] Live snapshots of a single block
> device
>
> On 05/22/2014 04:31 PM, Andrew Martin wrote:
> > Hello,
> >
>
> [Can you convince your mailer to wrap long lines?]
Sorry, my client won't auto-wrap and I forgot to run my post through vim
before
sending. Will do now :)
>
> > I am working on a script to automatically create live snapshots of
running
> > VMs using qemu-kvm 1.4.0 and libvirt 1.0.2. If a VM has
>
> Any reason you aren't upgrading to newer versions?  Although these seem
> sufficient for what you are trying.
The VM hosts I'm using all run Ubuntu 12.04 and I haven't had time to
backport
and test newer versions yet. I've published the latest versions I built in
this
PPA: https://launchpad.net/~xespackages/+archive/virtualization

The main functionality I need is live snapshotting with snapshot-create-as and
blockpull, so I haven't had a good reason to invest time to
upgrade.
>
> The problem you are facing is that the snapshot code MUST have an action
for
> every disk; and if you don't specify the action directly in your
> <domainsnapshot> xml, then the action chosen is inherited from the
<domain>
> xml (if you used <disk snapshot='no'>) or ultimately from the
type of snapshot
> you are creating (here, since you didn't use <disk
snapshot='no'> in the
        > domain xml, and didn't specify vdb in the
<domainsnapshot>, libvirt
        > assumes from --disk-only that you want vdb to have an external
        > snapshot taken).
>
> > </domainsnapshot>
> >
> > What am I doing wrong - how can I tell snapshot-create-as to create an
> > external snapshot for a specific block device only (not all block
devices)?
>
> Use: --diskspec vda --diskspec vdb,snapshot=no
>
> which produces this subset of xml:
>
>   <disks> <disk name='vda'/> <disk
name='vdb' snapshot='no'/> </disks>
>
Ah, that makes sense, thanks so much for clearing this up!
> >
> > Also, while looking at the manpage, does the --live option do anything
> > different if used with the above command?
>
> --live only makes sense when mixed with memory snapshots (with --memspec);
but
> as you are doing a --disk-only snapshot, it doesn't help (I'm not
sure if it
> will error out as mutually exclusive or just be silently ignored,
> without reading the code).
I'm using this code in a script for creating live backups of VMs - would it
make
sense to include --memspec to capture the memory state so that an fsck on boot
isn't necessary? I remember you explained --quiese awhile back:
https://www.redhat.com/archives/libvirt-users/2013-February/msg00020.html

However I don't have qemu-guest-agent installed on the guests, so would
using
--memspec instead of --disk-only produce a snapshot that is consistent (since it
would "resume" with the same memory state) or would this not be ideal
for backups (e.g easy to restore/recover)? Can the memory image be written to
the same image file (qcow2) as the disk snapshot?

Thanks,

Andrew