OS X 10.9 -> 10.10 upgrade headaches

Last month we moved the last production and test severs to Yosemite. (Yes, I know most of the readers of this blog made the transition many months ago, but I am talking about production servers that run financial and mobile operator software, not about bleeding edge developer computers). Some of these server (mostly older models Mac minis and Mac Pros used in the test environment) had problem upgrading.

Symptom: 
During upgrade from Mac OS 10.9 to 10.10 the upgrade process may hang for indefinite time but the server may remain accessible via ssh.


Solution: 
Install all the latest updates of 10.9 and then clean the nvram. In case the machine is hanging already a restart will return into the old 10.9 system. The command for cleaning nvram:

   sudo nvram -c
After cleaning the nvram, restart the machine again and then try upgrading the operating system


Note that after the nvram cleanup one may need to restore some settings there, for example the VM compression:
   sudo nvram boot-args="vm_compressor=1"
After this (and probably other nvram settings) the machine will need a reboot again.

Thanks to our Sys Admin (Georgi Karageorgiev) for finding the solution.