I had an interesting question from the field that warranted a response. The question, in the simplest terms, was:
"If I have two CRM / web servers (with all server roles installed) behind a load balancer, I'll cut one out from the load balancer to install an Update Rollup. As the Update Rollup install will be updating the CRM organizational and configuration databases that the remaining CRM / web server is accessing, will my users still hitting the remaining online CRM server be affected during the Update Rollup installation?"
In short, the answer is no, they will not be affected. They can continue using the system while servers are being updated. Database updates are backward compatible so that users hitting the not-yet-upgraded web server after the database is updated will be fine.
All dbUpdates are designed to be forward/backward compatible because in multi-server environments, as is commonly the case with our Enterprise customers, you cannot guarantee all serviced components are at the same patch level. In fact, practically speaking, this is impossible. This forward/backward compatibility pertains to not only database updates, but any change that affects more than one "Serviced Component". "Serviced Components" consist of:
All installers (MSIs):
This forward/backward compatibility applies whether the Update Rollup is un-installable or not un-installable.
Greg NicholsCRM Premier Field EngineeringMicrosoft Corporation
This conflicts with what's described in the Technet article "Updating Microsoft Dynamics CRM 2011" : technet.microsoft.com/.../hh699658.aspx
In what order should I apply an update rollup?
For server roles and server role groups, you must apply the same update rollup to them before you bring them back online. Similarly, if you have Microsoft Dynamics CRM Server 2011 installed on multiple computers, such as in a Network Load Balancing (NLB) configuration, you must update all the nodes in the NLB cluster before you bring them back online. A mixed deployment of Microsoft Dynamics CRM servers or server roles that are running different update rollup versions is not supported.