Virtual Machine Migration

VM migration in Private Cloud Director refers to the process of moving a running or stopped virtual machine (VM) from one compute host (hypervisor) to another within the same Private Cloud Director environment. This is a fundamental capability for maintaining system availability, performing infrastructure maintenance, and balancing workloads in data centers.

Common Use Cases

Following are the common scenarios in which migration is initiated in Private Cloud Director.

• Maintenance mode: When you invoke Maintenance Mode on host, VMs are live migrated to other compatible hosts in the virtualized cluster.
• Load Balancing: Private Cloud Director Dynamic Resource Rebalancing (DRR) uses live migration to distribute VMs more evenly across hosts in a virtualized cluster to optimize resource utilization and prevent individual hosts from becoming overloaded.
• Host Failure (VM Evacuation): Private Cloud Director Virtual Machine High Availability (VM HA) service uses VM evacuation to recover VMs from a failing or failed host.

Types of VM Migration

Private Cloud Director supports follwing types of VM migrations:

• Cold migration
• Live migration
• VM evacuation

Lets dive into the specifics of each.

Cold Migration

Cold migration is a method of moving a virtual machine from one host to another, where the VM is shut down (powered off)or suspended during the process. The VM disks and configuration files are transferred to the destination host as part of the cold migration process. ** Downtime: Cold migration essentially requires VM downtime as the VM needs to be powered off or suspended before it can be cold migrated. **

Cold Migration Prerequisites

• VM should be powered off or suspended.
• Cold migration uses the management network for copying over virtual machine disk information. Make sure that the management network interface has sufficient network bandwidth and capacity to support disk copy operation.
• All hosts within the virtualized cluster must meet CPU Model Pre-requisites.

Using CLI

1. Authenticate pcdctl CLI
2. Run the below command to perform the cold migration. The command below will cold migrate the VM to a suitable host within the virtualized cluster.

$ pcdctl server migrate <VM_UUID>

3. To migrate VM to a specific host use the below command:

$ pcdctl server migrate --host <HOST_UUID> <VM_UUID>

Using UI

• Log in to the UI and select Virtual Machine menu on the left side nav bar.
• Select the desired VM then click on the "other" option on the action bar and select the migrate action.
• Select the target hypervisor host and hit the "Migrate VM" button.

Live Migration

Live migration is a method of moving a virtual machine from one host to another, where the VM remains running throughout the migration process.

The live migration process copies a virtual machine's memory from source to destination host while the VM is running. Any memory pages that get modified or "dirtied" on the VM during this time are copied over again. Finally the VM enters a brief pause period during which it's remaining memory and CPU state are copied over to the destination host and finally the VM is resumed on the destination host. ** Downtime: The VM will experience a very short downtime (typically milliseconds to seconds) during the pause period before it resumes on the destination host.

Live migration can be classified further by the way it treats virtual machine storage:

• Live Migration for a VM using Ephemeral Shared Storage: The virtual machine has an ephemeral root disk that is located on Ephemeral Shared Storage.

  • Live migration is able to test and validate that the source and target hosts are using the same underlying shared storage.
  • When validated, live migration is performed without copying over VM disk.

• Live Migration of a VM using Ephemeral Local Storage: Virtual machine has an ephemeral root disk that is located on Ephemeral Local Storage.

  • Live migration will copy over entire virtual machine root disk from source host to destination host.

• Live Migration of a VM using Volumes only: Virtual machine is using block storage volumes, rather than ephemeral disk.

  • Live migration will migrate the VM without performing storage copy. The same volumes will continue to be attached to the VM once it's migrated to the target host.

Live Migration Prerequisites

• Live migration uses the management network for copying over virtual machine memory and optionally disk information. Make sure that the management network interface has sufficient network bandwidth and capacity to support VM memory migration traffic.

  • Support for adding a separate network interface for live migration is coming in the future.

• When using live migration in a production setup, specifically for features like Dynamic Resource Rebalancing (DRR), it is recommended to configure all virtual machines to use shared storage only for best performance of migration operation.

• When using Ephemeral Local Storage, live migration requires sufficient network bandwidth and capacity to support virtual machine disk copy.

  • This is not required when using Ephemeral Shared Storage at virtualized cluster level or using Block Storage Volumes exclusively for all virtual machines.

• All hosts within the virtualized cluster must meet CPU Model Pre-requisites for live migration to work. Read more about CPU Models.

Using CLI

1. Authenticate pcdctl CLI
2. Run the below command to perform the live migration. Note: Here target host UUID is mandatory.

pcdctl server migrate --live-migration <VMUUID> --host <HOST_UUID>

Using UI

1. Log in to the PCD UI and select Virtual Machine section on the left side of the window.
2. In Virtual Machine tab, search for desired VM and select that VM
3. Click on the "other" tab and select the migrate option
4. This will list down the eligible target hypervisors for Live migration. Select the target hypervisor and hit the "Migrate VM" button.

VM Evacuation

VM Evacuation is specifically designed for disaster recovery. If a host fails or becomes unresponsive, VM evacuation attempts to restart a VM that was running on the failed host onto a healthy hosts.

VM Evacuation Prerequisites

• VM evacuation can only be performed for VMs using Ephemeral Shared Storage or Block Storage Volumes. If the VM is using Ephemeral Local Storage, evacuation can not work as virtual machine disk can not be copied over from the source host to a target host.
• All hosts within the virtualized cluster must meet CPU Model Pre-requisites for VM evacuation to work. Read more about CPU Models.

Note that VM evacuation operation can only be performed using the command line today.

Using CLI

1. Authenticate pcdctl CLI
2. Run the below command to perform the live migration. Note: Here target host UUID is mandatory.

$ pcdctl server evacuate --host <FAILED_HOST UUID>

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