Abstract : In a virtualized datacenter, the Service Level Agreement for an application restricts the Virtual Machines (VMs) placement. An algorithm is in charge of maintaining a placement compatible with the stated constraints. Conventionally, when a placement algorithm computes a schedule of actions to re-arrange the VMs, the constraints ignore the intermediate states of the datacenter to only restrict the resulting placement. This situation may lead to temporary constraint violations. In this thesis, we present the causes of these violations. We then advocate for continuous placement constraints to restrict also the action schedule. We discuss why the development of continuous constraints requires more attention but how the extensible placement algorithm BtrPlace can address this issue.