There a several tools available for upgrading hosts. You can use different upgrade tools based depending on the type of host you are upgrading (ESX or ESXi) and whether the hosts are managed by vCenter Server.

In-place upgrades of hosts from ESXi 4.0 to ESXi 4.1 are supported using the vihostupdate command-line utility. vihostupdate requires the vSphere Command-Line Interface (vSphere CLI).

In-place upgrades of ESX 4.0 to ESX 4.1 are supported using the vihostupdate command-line utility and the esxupdate command-line utility.

Upgrades from ESX/ESXi 3.5 and ESX/ESXi 4.0 to ESX/ESXi 4.1 are also supported using vCenter Update Manager. See the VMware vCenter Update Manager Installation and Adminstration Guide accessible on the VMware vCenter Update Manager Documentation page: http://www.configuresoft.net/support/pubs/vum_pubs.html.

Caution

If you are upgrading hosts managed by vCenter Server, you must upgrade to vCenter Server before you upgrade ESX/ESXi. If you do not upgrade in the correct order, you can lose data and lose access to your servers.

When you upgrade a host from ESX 3.x to ESX 4.x, the customized partitions in 3.x are not replicated in 4.x.