OK, so we're all pretty much in agreement that true 0% downtime is not achievable. Some have suggested that maybe we need to change the way we measure downtime. This is not just a fancy cop-out, but a recognition that 'downtime' at midnight may not really be downtime because the system is idle. I would like to add to that the idea that planned downtime may not really be downtime. I'm familiar with a few industries where full plant shutdowns are implemented for maintenance and upgrades and nobody counts this as downtime! Presumably, the expense and lost business are more than recovered through more stable, more efficient, and/or higher capacity systems.
What we really need is a way to 'profile' our downtime that takes into account user requirements, business requirments, presumed advantages associated with maintenance and upgrades, workload balancing, etc. Any ideas?