stoney conductor: VM Backup

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-Image-Snapshot with the Live-Image
• Retain: Export the snapshot files

A more detailed and technical description for these three sub-processes can be found here.

Furthermore there is an control instance, which can independently call these three sub-processes for a given machine. Like that, the stoney cloud is able to handle different cases:

Backup a single machine

The procedure for backing up a single machine is very simple. Just call the three sub-processes (snapshot, merge and retain) one after the other. So the control instance would do some very basic stuff:

object machine = args[0];
 
if snapshot( machine ):
  if merge( machine ):
    if retain( machine ):
      printf("Successfully backed up machine %s\n", machine);
    else:
      printf("Error while retaining machine %s: %s\n", machine, error);
  else:
    printf("Error while merging machine %s: %s\n", machine, error);
else:
  printf("Error while snapshotting machine %s: %s\n", machine, error);

Backup multiple machines at the same time

For each machine call snapshot

For each machine call merge -> retain

...

Sub-Processes

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
       • Important: Remember: The live-disk-image /path/to/images/vm-001.qcow2 still contains a reference to this file!! So you cannot delete or move it!!!
     • 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

Please note: After these steps, the live-disk-image /path/to/image/vm-001.qcow2 no longer contains a reference to the image at the retain location (/path/to/retain/vm-001.qcow2). This is important for the retain process.

See also: Merge workflow

Retain

1. Move the all the files in from the retain directory (/path/to/retain/) to the backup directory (/path/to/backup/)
   1. Move the VMs state file to the backup directory

      • mv /path/to/retain/vm-001.state /path/to/backup/vm-001.state

   2. Move the VMs disk image to the backup directory

      • mv /path/to/retain/vm-001.qcow2 /path/to/backup/vm-001.qcow2

        • Please note: If the VM vm-001 has more than just one disk image, repeat this step for each disk image
   3. Move the VMs XML description file to the backup directory

      • mv /path/to/retain/vm-001.xml /path/to/backup/vm-001.xml

   4. Move the VMs backend entry file to the backup directory

      • mv /path/to/retain/vm-001.backend /path/to/backup/vm-001.backend

See also Retain workflow