When you install the vSphere Client, you can also install vSphere Host Update Utility. vSphere Host Update Utility is a standalone Microsoft Windows application recommended for smaller deployments with fewer than 10 ESX/ESXi hosts, without vCenter Server or Update Manager.

vSphere Host Update Utility provides an easy way to upgrade from a remote location, without a CD, and with minimum down time. The application upgrades the virtual machine kernel (vmkernel) and the service console, where present. vSphere Host Update Utility does not upgrade VMFS datastores or virtual machine guest operating systems.

You can use vSphere Host Update Utility to upgrade ESX 3.x to ESX 4.0 and ESXi 3.5 hosts to ESXi 4.0. You cannot use vSphere Host Update Utility to convert ESX hosts to ESXi hosts, or the reverse.

When you select a host to be upgraded, the tool performs an automated host compatibility check as a preupgrade step. The check verifies that each host is compatible with ESX 4.0/ESXi 4.0, including the required CPU, and has adequate boot and root partition space. In addition to the automated preupgrade script, you can specify a postupgrade configuration script to ease deployment. This tool effectively eliminates the need for complex scripted upgrades.

The Host Upgrade wizard allows you to make upgrade configuration choices. Your choices are saved in an installation script, which the software uploads to the selected host along with the installation ISO image or ZIP file. vSphere Host Update Utility supports local or remote mounted ISO binary image files, and ZIP files.

The software initiates the upgrade by rebooting the host and running the upgrade script. While an upgrade is in progress, vSphere Host Update Utility provides visual status so that you can monitor the status of the upgrade. If an error occurs during this process, the software rolls back the host software to the previous ESX version.

In addition to performing upgrades, vSphere Host Update Utility lets you learn about, download, and install maintenance and patch releases, which provide security, stability, and feature enhancements for ESXi 4.0.