list all the block devices associated with the guest

$ virsh domblklist testvm --detailsType       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.qcow2file format: qcow2virtual size: 20G (21474836480 bytes)disk size: 2.5Mcluster_size: 65536backing file: /export/vmimgs/testvm.qcow2$

created 2 more snapshots

$ virsh snapshot-list testvm --treesnap1-testvm  |  +- snap2-testvm      |      +- snap3-testvm

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

#--------------------------------------------#$ qemu-img info /export/vmimgs/snap3-testvm.qcow2image: /export/vmimgs/snap3-testvm.qcow2file format: qcow2virtual size: 20G (21474836480 bytes)disk size: 129Mcluster_size: 65536backing file: /export/vmimgs/snap2-testvm.qcow2#--------------------------------------------#$ qemu-img info /export/vmimgs/snap2-testvm.qcow2image: /export/vmimgs/snap2-testvm.qcow2file format: qcow2virtual size: 20G (21474836480 bytes)disk size: 3.6Mcluster_size: 65536backing file: /export/vmimgs/snap1-testvm.qcow2#--------------------------------------------#$ qemu-img info /export/vmimgs/snap1-testvm.qcow2image: /export/vmimgs/snap1-testvm.qcow2file format: qcow2virtual size: 20G (21474836480 bytes)disk size: 2.5Mcluster_size: 65536backing 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 --verboseBlock Pull: [100 %]Pull complete

可以看到snap3的backup为snap1

$ qemu-img info /export/vmimgs/snap3-testvm.qcow2image: /export/vmimgs/snap3-testvm.qcow2file format: qcow2virtual size: 20G (21474836480 bytes)disk size: 145Mcluster_size: 65536backing file: /export/vmimgs/snap1-testvm.qcow2

但snapshot-list 仍显示snap2

$ virsh snapshot-list testvm --treesnap1-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