Managing KVM/QEMU Disk Formats and Snapshots with qemu-img and virsh

This article demonstrates how to manage KVM/QEMU virtual machine disk images and snapshots on a Linux host.

Check Disk Format

cd /var/lib/libvirt/images/
qemu-img info ehs-rac-01.qcow2
image: ehs-rac-01.qcow2
file format: qcow2
virtual size: 30G (32212254720 bytes)
disk size: 34G
cluster_size: 65536
Snapshot list:
ID TAG VM SIZE DATE VM CLOCK
1 ehs-rac-01-bak 1.6G 2019-09-09 16:00:13 00:16:16.672
2 ehs-rac-01-bak02 1.9G 2019-09-09 16:04:40 00:20:27.294
Format specific information:
  compat: 1.1
  lazy refcounts: true

KVM uses raw format by default for best performance, but raw does not support features such as compression or encryption; qcow2 is required for those features. Since the image is already qcow2, conversion is unnecessary.

Convert Disk Format (if needed)

qemu-img convert -f raw -O qcow2 ehs-rac-01.img ehs-rac-01.qcow2

-f specifies the source format, -O the target format.

Snapshot Management

Create Snapshot

virsh snapshot-create ehs-rac-01

virsh snapshot-create-as ehs-rac-01 ehs-rac-01-bak

List Snapshots

virsh snapshot-list ehs-rac-01

Name               Creation Time               State
------------------------------------------------------------
ehs-rac-01-bak     2019-09-09 16:00:13 +0800   running
ehs-rac-01-bak02  2019-09-09 16:04:39 +0800   running

Show Current Snapshot

virsh snapshot-current ehs-rac-01

<domainsnapshot>
  <name>ehs-rac-01-bak02</name>
  <state>running</state>
  <parent>
    <name>ehs-rac-01-bak</name>
  </parent>
  <creationTime>1568016279</creationTime>
  <memory snapshot='internal'/>
  <disks>
    <disk name='hda' snapshot='internal'/>
    <disk name='hdb' snapshot='no'/>
  </disks>
  ...
</domainsnapshot>

Locate Snapshot Configuration Files

ls -l /var/lib/libvirt/qemu/snapshot/ehs-rac-01/

total 16
-rw------- 1 root root 5250 Sep 9 16:04 ehs-rac-01-bak02.xml
-rw------- 1 root root 5193 Sep 9 16:04 ehs-rac-01-bak.xml

Restore a Snapshot

First shut down the VM: virsh destroy ehs-rac-01 Confirm the VM is stopped, then revert to the desired snapshot (e.g., ehs-rac-01-bak02):

virsh snapshot-revert ehs-rac-01 ehs-rac-01-bak02

If you prefer to revert by timestamp, replace the snapshot name with the epoch time:

virsh snapshot-revert ehs-rac-01 1500373324

Delete a Snapshot

Identify existing snapshots with qemu-img info or virsh snapshot-list, then delete the unwanted one: virsh snapshot-delete ehs-rac-01 ehs-rac-01-bak Verify deletion:

virsh snapshot-list ehs-rac-01

Name               Creation Time               State
------------------------------------------------------------
ehs-rac-01-bak02  2019-09-09 16:04:39 +0800   running

The snapshot ehs-rac-01-bak has been removed.

For more details, see the original article.