The massive amount of resources found in datacenters makes it possible to provide high availability to multi-tier applications. Virtualizing these applications makes it possible to consolidate them on servers, reducing runtime costs. Nevertheless, replicated VMs have to be carefully placed within the datacenter to provide high availability and good performance. This requires resolving potentially conflicting application and datacenter requirements, while scaling up to the size of modern datacenters. We present Btrplace, a flexible consolidation manager that is customized through configuration scripts written by the application and datacenter administrators. BtrPlace relies on constraint programming and an extensible library of placement constraints. The present library of 14 constraints subsumes and extends the capabilities of existing commercial consolidation managers. Scalability is achieved by splitting the datacenter into partitions and computing placements in parallel. Overall, Btrplace repairs a non-viable placement after a major load increase or a maintenance operation for a 5,000 server datacenter hosting 30,000 VMs and involving thousands of constraints in 3 minutes. Using partitions of 2,500 servers, placement computing is reduced to under 30 seconds.