Real-time Maintenance Windows Should Not Affect Your Customers. If you design your hardware stack properly (see the model described in the Internet Delivery Stack) and make use of redundant hardware, you can still maintain full production availability even during a maintenance window.
Real-time maintenance allows you to roll out new features more quickly as you avoid having to schedule and announce maintenance windows to your customers. Your developers will appreciate it because they’re able to work more normal business hours, but in a real-time maintenance environment. Users like it too because their confidence in the reliability and stability of your entire application solution increases.
Excerpt from: e-Tips: Seven Essential Hosting Questions For SaaS Providers.