Linux KVM Virtualization Manual Low-Level Guest Export/Import Procedure

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Internal

Overview

The goal of a snapshotting operation is to preserve the state of the guest - its definition and the state of its storage - at a certain moment in time, so it can be reverted to, later, possibly after the guest was removed for a while from the virtualization host. This is one of the possible procedures to snapshot a KVM virtual machine and involves manually saving the XML definition of the guest and the state of its files systems. The guest must be down to take the snapshot. This procedure does not produce a formal snapshot, recorded by the virtualization host.

Another way to snapshot a guest is:

Guest Snapshot Using virsh snapshot Functionality

Take a Virtual Machine Snapshot


If the virtual machine has more than one storage device attached, they all must be snapshot at the same time to produce a consistent set of storage snapshots that can be reverted to.

Shutdown the Target Guest

virsh shutdown <vm-name>

Identify all Storage Devices

virsh dumpxml <vm-name>

Look for <disk> definitions. The disk definition specifies the source and the format of the virtual storage device. Example: 

<disk type='file' device='disk'>
    <driver name='qemu' type='qcow2'/>
    <source file='/main-storage-pool/appproxy.qcow2'/>
    ...
</disk>

Preserve the Content of all Storage Devices

The storage volumes can be snapshot in-place, but in this case the virtualization host's storage pool will continue to store the content, or they can be removed from the storage pool and archived off-line. Both procedures are available below:

Save the XML Definition

This is only necessary if we plan to modify the hardware configuration, or we want to temporarily remove any reference to the guest from the virtualization host.

Restore from Snapshot

Perform the steps described in the "Take a Virtual Machine Snapshot" section in reverse order:

qemu-img check https://kb.novaordis.com/index.php/Qemu-img#check

Restore the XML Definition

This is only necessary if the hardware configuration changed between snapshots.

Restore the Content of Storage Device from Snapshot

qemu-img snapshot -a <snapshot-tag> <volume-name>

For more details about restoration mechanics, see 

qemu-img snapshot -a
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