[libvirt-users] External snapshot issue

[Kashyap Chamarthy]

On Sat, Apr 22, 2017 at 11:26:17PM -0500, Leroy Tennison wrote:
> I have used
> 
>    virsh snapshot-create-as <VM name> <snapshot name> "<snapshot
>    description>" --diskspec
>    "vda,snapshot=external,file=/path/to/external-snapshot" --disk-only
>    --atomic

To avoid creating libvirt metadata for external snapshots, here you can
provide the option '--no-metadata' to `virsh snapshot-create-as` so that
libvirt won't track the metadata anymore.  This avoids having to delete
it manually (which you can do as I outlined further down below).

> to create an external snapshot of a running VM.  I followed it with
> 
>    virsh blockpull <VM name> --path /path/to/external-snapshot

Slightly related: `blockcommit` is relatively faster than `blockpull` in
this case -- because your base image ("original-image-file.qcow") should
have the bulk of the content, so `blockcommit` will copy only the
_delta_ from the original, which was stored in your
'external-snapshot.qcow2', so it'll be a shorter copy operation.

Here's some notes related to it:

    http://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit

> and monitored it until done.  I confirmed it with
> 
>    qemu-img info /path/to/external-snapshot
> 
> which shows no backing store.
> 
> However, when I do
> 
>    grep {/etc,/run}/libvirt/qemu/<VM name>.xml
> 
> the xml for the VM still shows a backing store in the /run/... definition
> (but not in the /etc/... definition, my understanding is that the /etc/...
> definition won't be updated until the VM is shut down):
> 
>       <disk type='file' device='disk'>
>         <driver name='qemu' type='qcow2'/>
>         <source file='/var/lib/libvirt/images/external-snapshot.qcow2'/>
>         <backingStore type='file' index='1'>
>           <format type='qcow2'/>
>           <source file='/var/lib/libvirt/images/original-image-file.qcow'/>
>           <backingStore/>
>         </backingStore>
>         <target dev='hda' bus='ide'/>
>         <alias name='ide0-0-0'/>
>         <address type='drive' controller='0' bus='0' target='0' unit='0'/>
>       </disk>
> 
> This is using libvirtd version 1.3.1 and qemu-img version 2.5.0.
> 
> How do I resolve this situation?  I don't want to have to rely on a backing
> store permanently.  Thanks for any help or pointers.

You can look at the existing snapshot metadata libvirt is tracking:

    $ virsh snapshot-list --tree vm1

And then delete the metadata (the below command does that) for a named
snapshot:

    $ virsh snapshot-delete vm1 --metadata snap1


-- 
/kashyap