When you upgrade hosts, you must understand and follow the best practices process for a successful upgrade.
For a successful ESXi upgrade, follow these best practices:
Make sure that you understand the ESXi upgrade process, the effect of that process on your existing deployment, and the preparation required for the upgrade.
If your vSphere system includes VMware solutions or plug-ins, make sure they are compatible with the vCenter Server version that you are upgrading to. See the VMware Product Interoperability Matrix at http://www.vmware.com/resources/compatibility/sim/interop_matrix.php.
Read Upgrade Options for ESXi 6.0 to understand the upgrade scenarios that are supported, and the options and tools that are available to perform the upgrade.
Read the VMware vSphere Release Notes for known installation issues.
Prepare the system for the upgrade.
Make sure that the current ESXi version is supported for the upgrade. See Upgrade Options for ESXi 6.0.
Make sure that the system hardware complies with ESXi requirements. See Upgrade Requirements and VMware Compatibility Guide at http://www.vmware.com/resources/compatibility/search.php. Check for system compatibility, I/O compatibility with network and host bus adapter (HBA) cards, storage compatibility, and backup software compatibility.
Make sure that sufficient disk space is available on the host for the upgrade.
If a SAN is connected to the host, detach the Fibre Channel system before continuing with the upgrade. Do not disable HBA cards in the BIOS.
Back up the host before performing an upgrade. If the upgrade fails, you can restore the host.
Depending on the upgrade option you choose, you might need to migrate or power off all virtual machines on the host. See the instructions for your upgrade method.
After the upgrade, test the system to ensure that the upgrade completed successfully.
Apply a host's licenses. See Applying Licenses After Upgrading to ESXi 6.0.
Consider setting up a syslog server for remote logging, to ensure sufficient disk storage for log files. Setting up logging on a remote host is especially important for hosts with limited local storage. vSphere Syslog Collector is included as a service in vCenter Server 6.0 and can be used to collect logs from all hosts. See Required Free Space for System Logging. For information about setting up and configuring syslog and a syslog server, setting up syslog from the host profiles interface, and installing vSphere Syslog Collector, see the vSphere Installation and Setup documentation.
If the upgrade was unsuccessful and you backed up the host, you can restore the host.