stoney conductor: VM Backup

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Overview

This page describes how the VMs and VM-Templates are backed-up inside the stoney cloud.

Basic idea

The main idea to backup a VM or a VM-Template is, to divide the task into three subtasks:

  • Snapshot: Save the machines state (CPU, Memory and Disk)
  • Merge: Merge the Disk-Snapshot with the live-image
  • Retain: Export the snapshot files

A more detailed and technical description for these three sub-processes can be found in the following sub-chapters.

Snapshot

  1. Create a snapshot with state:
    • If the VM vm-001 is running:
      • Save the state of VM vm-001 to the file vm-001.state (This file can either be created on a RAM-Disk or directly in the retain location. This example however saves the file to a RAM-Disk):
        virsh save vm-001 /path/to/ram-disk/vm-001.state
      • After this command, the VMs CPU and memory state is represented by the file /path/to/ram-disk/vm-001.state and the VM vm-001 is shut down.
    • If the VM vm-001 is shut down:
      • Create a fake state file for the VM:
        echo "Machine is not runnung, no state file" > /path/to/ram-disk/vm-001.state
  2. Move the disk image /path/to/images/vm-001.qcow2 to the retain location:
    mv /path/to/images/vm-001.qcow2 /path/to/retain/vm-001.qcow2
    • Please note: The retain directory (/path/to/retain/) has to be on the same partition as the images directory (/path/to/images/). This will make the mv operation very fast (only renaming the inode). So the downtime (remember the VM vm-001 is shut down) is as short as possible.
    • Please note2: If the VM vm-001 has more than just one disk-image, repeat this step for every disk-image
  3. Create the new (empty) disk image with the old as backing store file:
    qemu-img create -f qcow2 -b /path/to/retain/vm-001.qcow2 /path/to/images/vm-001.qcow2
    • Please note: If the VM vm-001 has more than just one disk-image, repeat this step for every disk-image
  4. Set correct ownership and permission to the newly created image:
    • chmod 660 /path/to/images/vm-001.qcow2
    • chown root:vm-storage /path/to/images/vm-001.qcow2
    • Please note: If the VM vm-001 has more than just one disk-image, repeat these steps for every disk-image
  5. Save the VMs XML description
    • Save the current XML description of VM vm-001 to a file at the retain location:
      virsh dumpxml vm-001 > /path/to/retain/vm-001.xml
  6. Save the backend entry
    • There is no generic command to save the backend entry (since the command depends on the backend). Important here is, that the backend entry of the VM vm-001 is saved to the retain location: /path/to/retain/vm-001.backend
  7. Restore the VMs vm-001 from its saved state (this will also start the VM):
    virsh restore /path/to/ram-disk/vm-001.state
    • Please note: After this operation the VM vm-001 is running again (continues where we stopped it), and we have a consistent backup for the VM vm-001:
      • The file /path/to/ram-disk/vm-001.state contains the CPU and memory state of VM vm-001 at time T1
      • The file /path/to/retain/vm-001.qcow2 contains the disk state of VM vm-001 at time T1
      • The file /path/to/retain/vm-001.xml contains the XML description of VM vm-001 at time T1
      • The file /path/to/retain/vm-001.backend contains the backend entry of VM vm-001 at time T1
  8. Move the state file from the RAM-Disk to the retain location (if you used the RAM-Disk to save the VMs state)
    • mv /path/to/ram-disk/vm-001.state /path/to/retain/vm-001.state


See also: Snapshot workflow

Merge

  1. Check if the VM vm-001 is running
    • If not, start the VM in paused state:
      virsh start --paused vm-001
  2. Merge the live-disk-image (/path/to/images/vm-001.qcow2) with its backing store file (/path/to/retain/vm-001.qcow2):
    virsh qemu-monitor-command vm-001 --hmp "block_stream drive-virtio-disk0"
    • Please note: If a VM has more than just one disk-image, repeat this step for every image. Just increase the number at the end of the command. So command to merge the second disk image would be:
      virsh qemu-monitor-command vm-001 --hmp "block_stream drive-virtio-disk1"
  3. If the machine is running in paused state (means we started it in 1. because it was not running), stop it again:
    • virsh shutdown vm-001

Retain

  1. Move the files to the backup location:
    • Move the old disk image to the backup location and add the date as suffix to not overwrite older backups: mv my-vm-backup.qcow2 /path/to/backup/my-vm-backup.qcow2.date
    • Move the state file to the backup location and add the date as suffix to not overwrite older backups: mv my-vm.state /path/to/backup/my-vm.state.date

State of the art

Next steps