HA agent can’t be installed on virtual ESXi 4.1 hosts
Lately when I was trying to build my vLab for the upcoming VCAP exams, I had a little challenge to get HA working on my two node cluster. Because this vLab is running on top of my existing vSphere 5 environment, I have assigned no more than 2GB of vRAM to each ESXi 4.1 host.
Installation and the initial configuration went all fine, until I tried to add the ESXi host to the vCenter Server; I received a warning message “Cannot complete the configuration of the HA agent on the host”.
Okay, that’s not much information but after a quick look in the Task & Events tab I saw that the installation of the HA agent won’t start because the ramdisk can’t be created, see the screenshot below.
The next step I have taken is a quick look at the ESXi host itself, when installing the HA agent you can track the installation process by viewing the /var/log/vmware/vpx-iupgrade.log.
As mentioned above here, the vCenter gives an error message that the ramdisk can’t be created. On the ESXi host you can see the same message but with some extra interesting information, it states that there is no more space to create the ramdrive.
Okay, that’s cool because according to the VMware documentation 2GB should be enough. However VMware says this in there documentation (ESXi Installable and vCenter Server Setup Guide, page 13): “2GB RAM minimum. For upgrades, 3GB RAM is required if the ESXi host is managed by vCenter Server”.
So is it 2GB or 3GB when the ESXi host is managed by the vCenter Server? Well when I see the above error I will believe it should be more than 2GB 
So I shutdown the ESXi hosts and add 1GB extra to the virtual machine, so in total the virtual ESXi hosts will have 3GB of RAM available.
Now that the ESXi hosts have more RAM I tried to add the hosts to the vCenter Server and the HA/DRS Cluster, and it actually worked, the hosts are now added to vCenter and the HA agent is active 