First of all make sure your kernel is 2.6.32. If it is not, it happened to me that auto-upgrade failed for some reason (Because you are running a BPO kernel for example). In this case, depending on your architecture :
aptitude install linux-image-2.6-vserver-amd64
Given that the above kernel is used, you may proceed with the system upgrade. In similar situations i preffered to upgrade VMs first and then the host itself.
For the VMs the procedure would be:
- Update the /etc/apt/sources.list for squeeze
Do the APT update and download the packages (especially when doing this from the internet, assuming there is no local repository mirror):
sudo apt-get clean && sudo aptitude update
sudo aptitude full-upgrade --download-only
Make sure that you install the appropriate driver packages for your hardware. It would be unpleasant to find yourself cut off from your remote server :)
sudo aptitude install firmware-linux-nonfree
Do the safe upgrade for a few times until there is nothing left:
sudo aptitude safe-upgrade
Finally do a full upgrade:
sudo aptitude full-upgrade
For the host of your virtual machines there will be an additional step in updating GRUB to GRUB2. You can check if your bootloader was installed properly by running:
If you do not see any errors, it is most likely safe to reboot.
If you get ANY error message about missing drivers, DO NOT REBOOT. Install necessary drivers before rebooting.
Depending on your actual environment and setup the above steps might differ a little, but these should give you a general idea.