Chapter 13. Saving and restoring virtual machine state by using snapshots

• Your host is using RHEL 9.4 or later.
• The VM is using file-based storage.

• You create the VM snapshot only in one of the following scenarios:

  • The VM is shut-down.
  • If the VM is running, you use the --disk-only --quiesce options or the --live --memspec options.

• Example output of a supported snapshot:

  <domainsnapshot>
    <name>sample-snapshot-name-1<name>
    <state>shutoff</state>
    <creationTime>1706658764</creationTime>
    <memory snapshot='no'/>
    <disks>
      <disk name='vda' snapshot='external' type='file'>
        <driver type='qcow2'/>
        <source file='/var/lib/libvirt/images/vm-name.sample-snapshot-name-1'/>
      </disk>
    </disks>
    <domain type='kvm'>
    [...]

• Example output of an unsupported snapshot:

  <domainsnapshot>
    <name>sample-snapshot-name-2</name>
    <state>running</state>
    <creationTime>1653396424</creationTime>
    <memory snapshot='internal'/>
    <disks>
      <disk name='vda' snapshot='internal'/>
      <disk name='sda' snapshot='no'/>
    </disks>
    <domain type='kvm'>
    [...]

Prerequisites

• Your host is using RHEL 9.4 or later.

• The VM uses file-based storage. To check whether this is the case, use the following command and ensure that for the disk device, it displays disk type as file:

  # virsh dumpxml <vm-name> | grep "disk type"
      <disk type='file' device='disk'>
      <disk type='file' device='cdrom'>

• If you want to create a VM snapshot that includes the memory of a running VM, you must have sufficient disk space to store the memory of the VM.

  • The minimum recommended space for saving the memory of a VM is equal to the VM’s assigned RAM. For example, saving the memory of a VM with 32 GB RAM requires up to 32 GB of disk space.
  • If the VM is under heavy I/O load, significant additional disk space might be required.
  • If the VM has assigned VFIO passthrough devices, additional disk space might be required.

  • If a snapshot is created without pausing the VM, additional disk space might be required.

    Warning

    Red Hat recommends not saving the memory of a running VMs that is under very high workload or that uses VFIO passthrough devices. Saving the memory of such VMs might fill up the host disk and degrade the system. Instead, consider creating snapshots without memory for such VMs.

    In addition, note that not all VFIO devices are capable of creating snapshot with memory. Currently, creating a snapshot with memory works correctly only if the attached VFIO device is a Mellanox VF with the migration capability enabled.

Procedure

• To create a VM snapshot with the required parameters, use the virsh snapshot-create-as command.

  # virsh snapshot-create-as <vm-name> <snapshot-name> <optional-description> <additional-parameters>

  • To create a snapshot of a shut-down VM, use the --disk-only parameter. For example, the following command creates Snapshot1 from the current disk-state of the shut-down Testguest1 VM:

    # virsh snapshot-create-as Testguest1 Snapshot1 --disk-only
    Domain snapshot Snapshot1 created.

  • To create a snapshot that saves the disk-state of a running VM but not its memory, use the --disk-only --quiesce parameters. For example, the following command creates Snapshot2 from the current disk state of the running Testguest2 VM, with the description clean system install:

    # virsh snapshot-create-as Testguest2 Snapshot2 "clean system install" --disk-only --quiesce
    Domain snapshot Snapshot2 created.

  • To create a snapshot that pauses a running VM and saves its disk-state and memory, use the --memspec parameter. For example, the following command pauses the Testguest3 VM and creates Snapshot3 from the current disk and memory state of the VM. The VM memory is saved in the /var/lib/libvirt/images/saved_memory.img file. When the snapshot is complete, the VM automatically resumes operation.

    # virsh snapshot-create-as Testguest3 Snapshot3 --memspec /var/lib/libvirt/images/saved_memory.img
    Domain snapshot Snapshot3 created.

    Pausing the VM during the snapshot process creates downtime, but might work more reliably than creating a live snapshot of a running VM (by using the --live option), especially for VMs under a heavy load.

  • To create a snapshot that saves the disk-state of a running VM as well as its live memory, use the --live --memspec parameters. For example, the following command creates Snapshot4 from the current disk and memory state of the running Testguest4 VM, and saves the memory state in the /var/lib/libvirt/images/saved_memory2.img file.

    # virsh snapshot-create-as Testguest4 Snapshot4 --live --memspec /var/lib/libvirt/images/saved_memory2.img
    Domain snapshot Snapshot4 created.

Prerequisites

• Your host is using RHEL 9.4 or later.
• The web console VM plug-in is installed on your system.

• The VM uses file-based storage. To ensure that this is the case, perform the following steps:

  1. In the Virtual machines interface of the web console, click the VM of which you want to create a snapshot.
  2. In the Disks pane of the management overview, check the Source column of the listed devices. In all devices that list a source, this source must be File.

Prerequisites

• A snapshot of the VM is available, which you have previously created in the web console or by using the command-line interface.
• Optional: You have created a snapshot of the current state of the VM. If you revert to a previous snapshot without saving the current state, changes performed on the VM since the last snapshot will be lost.

Procedure

• Use the virsh snapshot-revert utility and specify the name of the VM and the name of the snapshot to which you want to revert. For example:

  # virsh snapshot-revert Testguest2 clean-install
  Domain snapshot clean-install reverted

Verification

• Display the currently active snapshot for the reverted VM:

  # virsh snapshot-current Testguest2 --name
  clean-install

Prerequisites

• The web console VM plug-in is installed on your system.
• A snapshot of the VM is available, which you have previously created in the web console or by using the command-line interface.
• Optional: You have created a snapshot of the current state of the VM. If you revert to a previous snapshot without saving the current state, changes performed on the VM since the last snapshot will be lost.

Verification

• In the Snapshots pane, if a green check symbol now displays on the left side of the selected snapshot, you have successfully reverted to it.

Prerequisites

• Optional: A child snapshot exists for the snapshot you want to delete.

  A child snapshot is created automatically when you have an active snapshot and create a new snapshot. If you delete a snapshot that does not have any children, you will lose any changes saved in the snapshot after it was created from its parent snapshot.

  To view the parent-child structure of snapshots in a VM, use the virsh snapshot-list --tree command. The following example shows Latest-snapshot as a child of Redundant-snapshot.

  # virsh snapshot-list --tree <vm-name>
  
  Clean-install-snapshot
    |
    +- Redundant-snapshot
        |
        +- Latest-snapshot

Procedure

• Use the virsh snapshot-delete command to delete the snapshot. For example, the following command deletes Redundant-snapshot from the Testguest1 VM:

  # virsh snapshot-delete Testguest1 Redundant-snapshot
  Domain snapshot Redundant-snapshot deleted

Verification

• To ensure that the snapshot that you deleted is no longer present, display the existing snapshots of the impacted VM and their parent-child structure:

  # virsh snapshot-list --tree <Testguest1>
  
  Clean-install-snapshot
    |
    +- Latest-snapshot

  In this example, Redundant-snapshot has been deleted and Latest-snapshot has become the child of Clean-install-snapshot.

Prerequisites

• The web console VM plug-in is installed on your system.

• Optional: A child snapshot exists for the snapshot you want to delete.

  A child snapshot is created automatically when you have an active snapshot and create a new snapshot. If you delete a snapshot that does not have any children, you will lose any changes saved in the snapshot after it was created from its parent snapshot.

  To check that the snapshot has a child, confirm that the snapshot is listed in the Parent snapshot column of the Snapshots in the web console overview of the VM.

Verification

• If the snapshot no longer appears in the Snapshots pane, it has been deleted successfully.