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About cold and warm migration

{project-short} supports cold migration from:

  • VMware vSphere

  • {rhv-full} ({rhv-short})

  • {osp}

  • Remote {virt} clusters

{project-short} supports warm migration from VMware vSphere and from {rhv-short}.

Cold migration

Cold migration is the default migration type. The source virtual machines are shut down while the data is copied.

Warm migration

Most of the data is copied during the precopy stage while the source virtual machines (VMs) are running.

Then the VMs are shut down and the remaining data is copied during the cutover stage.

Precopy stage

The VMs are not shut down during the precopy stage.

The VM disks are copied incrementally by using changed block tracking (CBT) snapshots. The snapshots are created at one-hour intervals by default. You can change the snapshot interval by updating the forklift-controller deployment.

Important

You must enable CBT for each source VM and each VM disk.

A VM can support up to 28 CBT snapshots. If the source VM has too many CBT snapshots and the Migration Controller service is not able to create a new snapshot, warm migration might fail. The Migration Controller service deletes each snapshot when the snapshot is no longer required.

The precopy stage runs until the cutover stage is started manually or is scheduled to start.

Cutover stage

The VMs are shut down during the cutover stage and the remaining data is migrated. Data stored in RAM is not migrated.

You can start the cutover stage manually by using the {project-short} console or you can schedule a cutover time in the Migration manifest.

Advantages and disadvantages of cold and warm migrations

Detailed description

The table that follows offers a more detailed description of the advantages and disadvantages of each type of migration. It assumes that you have installed Red Hat Enterprise Linux (RHEL) 9 on the {ocp} platform on which you installed {project-short}.

Table 1. Detailed description of advantages and disadvantages
Cold migration Warm migration

Fail fast

Each VM is converted to be compatible with {ocp-short} and, if the conversion is successful, the VM is transferred. If a VM cannot be converted, the migration fails immediately.

For each VM, {project-short} creates a snapshot and transfers it to {ocp}. When you start the cutover, {project-short} creates the last snapshot, transfers it, and then converts the VM.

Tools

{project-short} only.

{project-short} and CDI from {virt}.

Parallelism

Disks must be transferred sequentially.

Disks can be transferred in parallel using different pods.
Note

The preceding table describes the situation for VMs that are running because the main benefit of warm migration is the reduced downtime, and there is no reason to initiate warm migration for VMs that are down. However, performing warm migration for VMs that are down is not the same as cold migration, even when {project-short} uses virt-v2v and RHEL 9. For VMs that are down, {project-short} transfers the disks using CDI, unlike in cold migration.
Note

When importing from VMware, there are additional factors which impact the migration speed such as limits related to ESXi, vSphere. or VDDK.

Conclusions

Based on the preceding information, we can draw the following conclusions about cold migration vs. warm migration:

  • The shortest downtime of VMs can be achieved by using warm migration.

  • The shortest duration for VMs with a large amount of data on a single disk can be achieved by using cold migration.

  • The shortest duration for VMs with a large amount of data that is spread evenly across multiple disks can be achieved by using warm migration.