Klaus,
Depending on the applications you're running, you may need a large
number of inodes. In this case (assuming memory isn't a problem) you
can just keep increasing inode-max until you find a limit you can live
with.
It's possible (though unlikely I hope) that somehow inode use counts
aren't being released, and that effectively there's an inode "leak". If
your usage keeps going up over time this would be something to consider.
Another possibility is that your applications are using large numbers of
inodes in a pattern that's defeating the free inode search algorithm.
If you're willing to experiment with this, I can send you a small patch
that changes the search mechanism.
Regards,
Bill