Master the differences of hot and cold migrations in VMware

Hot and cold migrations move VMs differently throughout VMware infrastructure. A hot migration doesn't require downtime, while a cold migration moves powered-off VMs.

Though hot and cold migrations move workloads to any desired location, they do so differently. Not only do hot and cold migrations move different types of data, each also uses a separate network type within vSphere.

To ensure the integrity of the data you must understand the differences between a hot and cold migration in VMware environments, their use cases and which networks to use.

What are hot and cold migrations?

A hot migration -- also known as a live migration -- is the process of migrating an active workload, such as a VM, from one physical machine to another. A vMotion-enabled service initiates a hot migration with zero downtime. This lets you replace or upgrade parts of the VM without a machine or system shutdown.

But hot migrations risk data corruption. A hot migration also can't live-migrate VMs between two systems with different CPUs.

A cold migration is you suspend or power off a VM and migrate it between hosts. You can also migrate associated disks from one datastore to another. Cold migrations move VMs between ESXi hosts with different CPU architectures.

Cold migrations have a less complex migration pathway. Cold migrations can lead to long downtimes because you must power down the system to initiate the process.

Network use types for hot and cold migrations

You can initiate a hot or cold migration for VMware setups via the vCenter Server UI or through an automation tool such as PowerCLI.

VSphere has several services you can enable on the VMkernel interface: vMotion, provisioning, fault tolerance logging, management, vSphere replication, vSphere replication NFC and vSAN. VSphere uses three of these enabled services for hot and cold migrations: vMotion, provisioning and management networks.

The vSphere VMkernel interface automatically enables the management service, but you can configure vSphere to enable other services. Hot migrations use the vMotion-enabled network to live-migrate workloads, while cold migrations use the provisioning network -- as long as you enable this service. If you don't enable the provisioning network for a cold migration; vSphere uses the default management network.

VMotion migrates any cold data, such as VM snapshots and nonchild delta disks, and use the provisioning network as long as you enable it. If you don't enable the provisioning network, vSphere uses the default management network to move cold data.

Make the choice between a hot versus cold migration

The choice between a hot or cold migration depends on the level of risk and time you're willing to take. Hot migrations enable you to live-migrate workloads and maintain full access to workloads as they move. This leaves VMs vulnerable to data corruption. Some physical machines, such as Active Directory controllers, aren't suited for hot migrations.

Cold migrations let you power off workloads and move them between hosts to prevent data loss -- but this leads to long downtimes. SQL servers that update regularly are better off using a cold migration.

Assess your business needs, time requirements and resources you need to complete a migration and you can better determine which migration type is best for your organization.

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