This scenario is known as a migration upgrade. The migration upgrade is a managed transition rather than a strict upgrade. By using VMotion to move virtual machines directly from one production host to another production host, you minimize downtime of the virtual machines.

The following example provides a high-level overview of the upgrade process in an environment with ESX 3.x/ESXi 3.5 and VirtualCenter 2.x, using VMotion to migrate your running virtual machines to ESX 4.0/ESXi 4.0. The hosts in your environment must be licensed for and able to use VMotion.

You can perform a migration upgrade without VMotion. The only difference is the amount of downtime for the virtual machines.

The disadvantage of a migration upgrade is that this plan requires additional resources. A migration upgrade calls for sufficient resources to run the production environment partly on older hosts and partly on upgraded hosts. Any required redundancies and safeguards must be available on both upgraded and non-upgraded infrastructure during the transition.

The requirements for a migration upgrade with VMotion are as follows:

One or more machines meeting ESX 4.0/ESXi 4.0 requirements.

Empty host storage sufficient to hold a portion of your production virtual machines. Ideally, the storage should be large enough to hold all of the migrated virtual machines. A larger capacity for virtual machines on this extra storage means fewer operations are required before all your virtual machines are migrated.

Before you begin this procedure, complete the following tasks:


Upgrade VirtualCenter 2.x to vCenter Server 4.0.


Make sure your database is compatible with vCenter Server 4.0. This release discontinues support for some database versions and adds support for other database versions. See the Compatibility Matrixes on the VMware vSphere documentation Web site.


Make sure that you have the required permissions to perform this procedure. See Database Prerequisites.


Take a full backup of the VirtualCenter 2.x database. See your database documentation.


Back up the VirtualCenter 2.x SSL certificates. See Back Up VirtualCenter 2.x.

The downtime required for this upgrade is based on the amount of data in the database. During this time, you cannot perform provisioning operations, such as cloning or creating virtual machines. The upgrade from VirtualCenter 2.5 is faster in comparison to the upgrade from VirtualCenter 2.0.x because of differences in changes to the database schema and the amount of data migration.

After the upgrade, the ESX hosts are automatically reconnected to vCenter Server 4.0. Your VMware High Availability (HA) and VMware Distributed Resource Scheduler (DRS) clusters are automatically reconfigured. (Check to ensure that the automatic reconfiguration is successful. In some cases, you might need to reconfigure the clusters manually.)

For a detailed description of the upgrade procedure, see Preparing for the Upgrade to vCenter Server and Upgrading to vCenter Server 4.0.


Run the vCenter Agent Preupgrade Check tool.


Install the vSphere Client.

You can install the vSphere Client on the same machine with your previous version of the VI Client. You must have the previous version of the VI Client to connect to previous versions of VirtualCenter and ESX/ESXi.

For a detailed description of the procedure, see Upgrade to the vSphere Client.


If your environment has vCenter Converter, upgrade it to the latest version.


If your environment has vCenter Guided Consolidation, complete the consolidation plan and then upgrade it to the latest version.


If your environment has vCenter Update Manager, upgrade it to the latest version.


Use VMotion to evacuate the virtual machines from the ESX 3.x/ESXi 3.5 hosts.


Upgrade to ESX 4.0/ESXi 4.0, or perform a fresh installation of ESX 4.0/ESXi 4.0.


Add the ESX 4.0/ESXi 4.0 host to vCenter Server.

For VMotion to work, the ESX 3.x/ESXi 3.5 and ESX 4.0/ESXi 4.0 hosts must be managed by the same vCenter Server instance.


Use VMotion to move virtual machine to the ESX 4.0/ESXi 4.0 host.


Upgrade your virtual machines:


If they are not already powered on, power on the virtual machines and upgrade to the latest version of VMware Tools. This upgrade allows you to use the new features of ESX 4.0./ESXi 4.0.


Power off the virtual machines and upgrade to the latest version of virtual hardware to take advantage of the new virtual hardware. vSphere 4.0 supports some earlier virtual hardware versions. See Basic System Administration.

The virtual machine upgrade process is different for ESX 4.0/ESXi 4.0. In earlier versions, you upgraded the virtual hardware upgrade before you upgraded VMware Tools. For ESX 4.0/ESXi 4.0, you upgrade VMware Tools before you upgrade the virtual hardware.

You can use either the vSphere Client or vCenter Update Manager to upgrade virtual machines. In a clustered environment, VMware recommends that you use vCenter Update Manager . See the vCenter Update Manager Administration Guide. If you are using the vSphere Client to upgrade virtual machines, see Upgrading Virtual Machines.


Upgrade your product licenses:


Either your new license keys are sent to you in email, or you get them using the license portal.


Apply the new license keys to your assets using the vSphere Client (or vCenter Server if you have it).

You must perform these tasks for each ESX 2.5.x host and the virtual machines on the hosts.