I have a private OpenVZ node in which I have been having disk io issues with when containers are running cpanel backups.

I understand you are supposed to be able to limit the io, on containers, using the ioprio function. A number from 1-7 sets the io priority of the container.
On OpenVZ's wiki, it's a bit unclear wehter 0 or 7 is the highest priority, so I have tried both ends of the spectrum, with basically no result.
Once a container begins backing up, it takes almost all of the disk i/o available to the node.
Am I missing something to get ioprio to function?
Which is the highest and lowest priority? 0,7?
What other tweaks can I make to reduce cpanels's backup io demands, from within the containers?

Any help appreciated