External (and Live) snapshots with libvirt

list all the block devices associated with the guest

$ virsh domblklist testvm --details
Type       Device     Target     Source
------------------------------------------------
file       disk       vda        /export/vmimgs/testvm.qcow2

let’s create a snapshot(disk-only) of the guest

$ virsh snapshot-create-as testvm snap1-testvm "snap1 description" \
  --diskspec vda,file=/export/vmimgs/snap1-testvm.qcow2 \
  --disk-only --atomic

– ‘–atomic’ just ensures either the snapshot is run completely or fails w/o making any changes

$ qemu-img info /export/vmimgs/snap1-testvm.qcow2
image: /export/vmimgs/snap1-testvm.qcow2
file format: qcow2
virtual size: 20G ( bytes)
disk size: 2.5M
cluster_size:
backing file: /export/vmimgs/testvm.qcow2
$ 

created 2 more snapshots

$ virsh snapshot-list testvm --tree

snap1-testvm
  |
  +- snap2-testvm
      |
      +- snap3-testvm
        

image file chain[ base<-snap1<-snap2<-snap3 ]:

#--------------------------------------------#
$ qemu-img info /export/vmimgs/snap3-testvm.qcow2
image: /export/vmimgs/snap3-testvm.qcow2
file format: qcow2
virtual size: 20G ( bytes)
disk size: 129M
cluster_size:
backing file: /export/vmimgs/snap2-testvm.qcow2
#--------------------------------------------#
$ qemu-img info /export/vmimgs/snap2-testvm.qcow2
image: /export/vmimgs/snap2-testvm.qcow2
file format: qcow2
virtual size: 20G ( bytes)
disk size: 3.6M
cluster_size:
backing file: /export/vmimgs/snap1-testvm.qcow2
#--------------------------------------------#
$ qemu-img info /export/vmimgs/snap1-testvm.qcow2
image: /export/vmimgs/snap1-testvm.qcow2
file format: qcow2
virtual size: 20G ( bytes)
disk size: 2.5M
cluster_size:
backing file: /export/vmimgs/testvm.qcow2
$
#--------------------------------------------#

Now, if we do not need snap2 any more, and want to pull all the data from snap1 into snap3, making snap1 as snap3’s backing file, we can do a virsh blockpulloperation as below:

$ virsh blockpull --domain testvm \
  --path /export/vmimgs/snap3-testvm.qcow2 \
  --base /export/vmimgs/snap1-testvm.qcow2 \
  --wait --verbose
Block Pull: [ %]
Pull complete

可以看到snap3的backup为snap1

$ qemu-img info /export/vmimgs/snap3-testvm.qcow2
image: /export/vmimgs/snap3-testvm.qcow2
file format: qcow2
virtual size: 20G ( bytes)
disk size: 145M
cluster_size:
backing file: /export/vmimgs/snap1-testvm.qcow2

但snapshot-list 仍显示snap2

$ virsh snapshot-list testvm --tree
snap1-testvm
  |
  +- snap2-testvm
      |
      +- snap3-testvm

所以仍可以恢复为

base <- snap123

External disk-snapshots(live) using RAW as original image: 

源disk image 不变为raw,而snapshot为qcow2

internal snapshots会导致源disk image变成qcow2