Where the OpenNebula VM disk snapshot backup will be stored?

It depends on your storage system.

When your nodes storage are shared, the snapshots are available in front-end. Otherwise, you will need to take a look at the node where the VMs is running.

Let say I have this storage using GlusterFS. This storage are shared between nodes and front-end.

So all disk snapshot are available in front-end.

It located at /var/lib/one/datastores/$ID

oneadmin@one-frontend ~
# tree /var/lib/one/datastores/100/1810/
/var/lib/one/datastores/100/1810/
├── deployment.0
├── disk.0
├── disk.0.snap
│   ├── 0
│   ├── 1
│   └── 2
└── disk.1

1 directory, 6 files

Those disk.0.snap is the directory where snapshot file stored. I have three snapshot files, as well as shown at Sunstone.

And it's have a same size because I repeat snapshotting without make any change in VM.

oneadmin@one-frontend ~
# du -h /var/lib/one/datastores/100/1810/disk.0.snap/* /var/lib/one/datastores/100/1810/disk.0
2.1G    /var/lib/one/datastores/100/1810/disk.0.snap/0
2.1G    /var/lib/one/datastores/100/1810/disk.0.snap/1
2.1G    /var/lib/one/datastores/100/1810/disk.0.snap/2
1.5G    /var/lib/one/datastores/100/1810/disk.0