vSphere Upgrade Center
Phase 1: Perform the upgrade of the VMware® vCenter Server™ along with VMware® vCenter™ Update Manager™. Before starting this phase, make note of any other VMware solutions associated with your environment, such as VMware vCloud® Director™ (vCD) and VMware® vCenter™ Site Recovery Manager™ (SRM), and use the VMware Product Interoperability Matrix to determine which versions of these solutions are compatible with vCenter Server 5.5. In-place upgrades to vCenter 5.5 are supported from vCenter versions 4.0 and above. Alternatively, you can choose to do a clean installation of vCenter 5.5.
Phase 2: Upgrade your vSphere hosts to vSphere 5.5. This may be done in a rolling manner, with individual hosts in a cluster successively updated after virtual machines are migrated using VMware vSphere® vMotion® to the other running hosts. By doing so, you can upgrade your vSphere hosts while avoiding virtual machine downtime. In-place host upgrades to ESXi version 5.5 are supported from vSphere hosts running ESX/ESXi 4.0, 4.1 and ESXi 5.0 and 5.1. Optionally, you may choose to do a clean installation of vSphere 5.5. Upgrade methods for upgrading your vSphere hosts include using the ESXi Installer, the ESXCLI command from within the ESXi Shell, vSphere Update Manager and Auto Deploy.
Phase 3: Complete the upgrade of your virtual machines by first upgrading VMware Tools, then optionally upgrading the virtual hardware. vSphere 5.5 provides an extended support matrix that allows older versions of VMware Tools and older virtual hardware versions to continue to run in a supported configuration on newer versions of ESXi. This extended support matrix facilitates vSphere upgrades by allowing you to upgrade your vCenter Server and ESXi hosts right away followed by a phased upgrade of your virtual machines over time. vSphere 5.5 supports running virtual machines with VMware Tools version 4.x and 5.x as well as Virtual Hardware versions 4, 7, 8, 9 and 10.
Virtual machines need to be powered off to be upgraded, so you should plan appropriate outage windows for this step. You can perform an orchestrated upgrade of virtual machines with vSphere Update Manager, which allows you to upgrade VMware Tools and virtual hardware at the same time.
Phase 4: The final step is to upgrade your datastores to VMware vSphere® VMFS version 5. This can be done with live virtual machines running on the datastores and do not require any virtual machine downtime. vSphere 5.x fully supports datastores with VMFS version 3, so the datastore upgrade may be scheduled at a time that’s convenient for you.
When upgrading VMFS 3 volumes to VMFS 5, the upgraded VMFS-5 volume retains the partition characteristics of the original VMFS-3 datastore. For large VMFS volumes on shared storage consider creating a new VMFS 5 volume and using Storage vMotion to migrate the data.