It becomes even more complex when you are using an application like Oracle's E-business suite. During the upgrades/patching, the application is unavailable to the end users and it is quite difficult to engineer a solution where all transactions could be routed to another database and then somehow captured and reapplied to the 11i database after the upgrade. We have tried it once in the past but because of the number of tables involved (thousands) and the complexity of oracle apps, we were only able to provide offline operations to just one part of the business. So, if you are using Oracle apps and consider downtime as either scheduled or unscheduled then it will be extremely difficult to implement 99.9999% availability. We have explored this opportunity extensively and discovered that even Oracle Corporation internally shuts down the apps during upgrades. Go figure ..