You may need to take a managed server offline for a number of reasons, such as a rolling upgrade of virtualization software, adding or testing connectivity to a new network, diagnosing an underlying hardware issue or adding connectivity to a new storage system. In XenCenter, you can take a server offline temporarily by placing it into Maintenance Mode. If the server is in a resource pool, when you place it into Maintenance Mode, all running VMs will be automatically migrated from it to another server in the same pool. If the server is the pool master, a new master will also be selected for the pool.
When Workload Balancing is enabled, the virtual machines running on that server will be automatically migrated to their optimal servers when available, based on Workload Balancing recommendations (performance data, your placement strategy, and performance thresholds).
While a server is Maintenance Mode, you cannot create or start any VMs on it.