What is vMotion?

vMotion is VMware's live migration technology that allows a running virtual machine to be moved from one ESXi host to another — with zero downtime and no interruption to the workloads running inside the VM.

How It Works

At its core, vMotion works by transferring the entire memory state and execution context of a running VM across the network to a destination host while the VM continues to run. The process happens in three broad phases:

1. Pre-copy (memory mirroring) vMotion begins copying the VM's active memory pages from the source host to the destination host over a dedicated vMotion network. While this is happening, the VM keeps running and dirtying pages. vMotion tracks which pages have changed and iteratively copies those changed pages in successive rounds, converging toward a point where very few pages are left to transfer.

2. Switchover (stun and commit) Once the memory delta is small enough, vMotion briefly pauses (stuns) the VM — typically for milliseconds — copies the final few dirty pages and the CPU state to the destination, and resumes execution on the new host. From the VM's perspective, almost nothing happened.

3. Cleanup The source host releases its resources for the VM, and vCenter updates its inventory to reflect the new home of the VM. Network traffic is redirected to the new host via a gratuitous ARP announcement.

Requirements

Requirement	Detail
Shared Storage	Both hosts must access the same datastore (SAN, NAS, or vSAN) — unless using Storage vMotion simultaneously
vMotion Network	A dedicated VMkernel port group with vMotion enabled on both hosts
Compatible CPUs	Source and destination must have compatible CPU feature sets — use EVC (Enhanced vMotion Compatibility) clusters to mask differences
vCenter	Required to orchestrate the migration (host-to-host vMotion requires vCenter)
VM Hardware	No unsupported devices connected (e.g. physical CD-ROM, USB passthrough)

Types of vMotion

Type	What Moves	Shared Storage Required
vMotion	VM compute (CPU + RAM)	Yes
Storage vMotion	VM storage (VMDK files)	No — moves between datastores
vMotion + Storage vMotion	Both simultaneously	No

Why It Matters

vMotion is the foundation of VMware's operational flexibility. It enables:

Zero-downtime host maintenance — evacuate a host before patching or rebooting it
Load balancing — DRS (Distributed Resource Scheduler) uses vMotion automatically to balance workloads across cluster hosts
Power management — DPM (Distributed Power Management) consolidates VMs and powers down idle hosts
Disaster avoidance — proactively move VMs away from a host showing hardware warnings

In a casino environment specifically, vMotion lets you patch and reboot production hosts during maintenance windows without taking down any VMs — critical when uptime of slot management, surveillance, and cage systems is non-negotiable.

📖 Ready to migrate a VM? See the step-by-step guide: How to Migrate a Virtual Machine in vSphere