I was planning a quick upgrade of a small cluster of ESX3 hosts to the latest and greatest yesterday. After upgrading vCenter, Update Manager and the vSphere Client, I set up an upgrade baseline in VMware vCenter Update Manager, took an ESX-host out of production and tried to remediate the host. After some thirty minutes, the task failed. No decent error was displayed in the ‘events’ view within the vSphere Client.

As I was wading through log files (/esx4-upgrade/ is a great place to start, so is /var/log/vmware/), I noticed something about the /boot partition being too small. I had finally encountered this dreaded drawback in upgrading ESX3 to ESX4. Sadly, not much can be done about it, although I did try some magic to resize the /dev/sda1 partition on which /boot was residing, but to no avail. Today, the three servers in the cluster will be receiving a clean installation of ESX.

So, if you are having trouble upgrading to ESX4 using Update Manager (or the Host Upgrade Utility, for that matter), please remember that it could very well be caused by a small boot partition. Start your troubleshooting endeavours at /esx4-upgrade/. Also, make sure you read up on the vSphere Upgrade Guide, because it gives some invaluable information on the /boot partition:

The default ESX 2.5.5 installation creates a /boot partition that is too small to enable
upgrades to ESX 4.0. As an exception, if you have a non-default ESX 2.5.5 installation on which at least 100MB
of space is available on the /boot partition, you can upgrade ESX 2.5.5 to ESX 3.x and then to ESX 4.0.