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How does QEMU migrate and copy virtual machines?

Created on 2025-03-24 17:20:40
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QEMU (Quick Emulator) is a popular open-source emulator and virtualizer that allows for the migration and copying of virtual machines (VMs). Migration refers to the process of moving a running VM from one physical host to another without downtime, while copying involves creating an exact replica of a VM on a different host.

Migration

Migration in QEMU can be achieved using two main methods: live migration and offline migration.

Live Migration

Live migration allows a running VM to be moved from one host to another with minimal downtime. This is particularly useful for maintenance tasks or load balancing. The process involves transferring the VM's memory, CPU state, and device states from the source host to the destination host.

Steps:

  1. Preparation: Ensure both the source and destination hosts have compatible configurations and sufficient resources.
  2. Memory Transfer: The VM's memory is incrementally transferred from the source to the destination host.
  3. State Synchronization: The CPU state and device states are synchronized between the hosts.
  4. Final Transfer and Cutover: The remaining memory and final state changes are transferred, and the VM is switched to run on the destination host.

Example:

qemu-system-x86_64 -enable-kvm -m 2048 -hda /path/to/disk.img -incoming "tcp:0:port"

On the destination host, you would start QEMU in listening mode to accept the incoming migration.

Offline Migration

Offline migration involves shutting down the VM, copying its disk image and configuration files to the new host, and then starting the VM on the new host.

Steps:

  1. Shutdown: Power off the VM on the source host.
  2. Copy Files: Copy the VM's disk image and configuration files to the destination host.
  3. Start VM: Start the VM on the destination host using the copied files.

Example:

scp /path/to/disk.img user@destination:/path/to/destination/
scp /path/to/config.xml user@destination:/path/to/destination/

On the destination host, start the VM using:

qemu-system-x86_64 -enable-kvm -m 2048 -hda /path/to/destination/disk.img

Copying Virtual Machines

Copying a VM involves creating an exact replica of the VM on a different host. This can be done by copying the VM's disk image and configuration files.

Steps:

  1. Prepare Source VM: Ensure the VM is powered off.
  2. Copy Files: Copy the VM's disk image and configuration files to the destination host.
  3. Adjust Configuration: Modify the configuration files if necessary to reflect any changes in network settings or resource allocations.
  4. Start VM: Start the VM on the destination host.

Example:

scp /path/to/disk.img user@destination:/path/to/destination/
scp /path/to/config.xml user@destination:/path/to/destination/

On the destination host, adjust the configuration file if needed and start the VM:

qemu-system-x86_64 -enable-kvm -m 2048 -hda /path/to/destination/disk.img

Tencent Cloud Services for VM Migration and Copying

For users leveraging Tencent Cloud, the Cloud Block Storage (CBS) and Virtual Machine (CVM) services can be utilized for efficient VM migration and copying. CBS provides scalable block storage that can be easily attached to CVM instances, facilitating the transfer of disk images. Additionally, Tencent Cloud's Cloud Migration Service can help automate and streamline the migration process, ensuring minimal downtime and data integrity.

Using Tencent Cloud's services, you can create snapshots of your VMs, which can then be used to create new instances in different regions or availability zones. This approach ensures that your VMs are backed up and can be quickly deployed in case of hardware failures or for scaling purposes.

Example:

  1. Create Snapshot: Use the Tencent Cloud console to create a snapshot of your VM's disk.
  2. Create New Instance: Use the snapshot to create a new CVM instance in the desired region or availability zone.
  3. Migrate Data: If necessary, migrate additional data or configurations to the new instance.

By utilizing Tencent Cloud's robust infrastructure and services, you can ensure efficient and reliable VM migration and copying processes.