I’ve been experiencing a problem with a very small VMware ESXi 4 host on which I run two VM’s for VMware vCenter Lab Manager management (a vCenter VM and a LM VM). I was rebuilding the environment, so I had both VM’s running on this host with (at that time) 3GB of RAM. When adding this host to vCenter, I got weird and cryptic error messages:

Cannot install the vCenter agent service. Unknown installer error.

I Googled my ass off, but never found an explanation or workaround. Finally, I decided to move the VM’s to another host, put it in maintenance mode, reboot it, and tried to add it to vCenter once more. For some reason or another, this worked!

Mike Laverick of RTFM Education posted a link to a VMware KB article exactly described what went wrong in this situation, and how to fix it. Like Mike, I find it funny that the solution is to migrate the running VM’s to another host and put more memory in the box:

This issue can occur if the ESXi host is low on RAM. The host can be low
on RAM either because it does not have enough physical memory allocated
to it, or because too many virtual machines are consuming
RAM at the same time.

If the host does not have enough physical memory allocated to it, you
can allocate more physical RAM:

 1. Power off the virtual machines on the host.
 2. Power off the ESXi host.
 3. Allocate more physical RAM to the host.
 4. Reboot the ESXi host.

In any case, I’m happy Mike found this KB article, because I now know that more (RAM) is always better, especially in this case! Thanks Mike!