When Workload Balancing is enabled, if you take a physical host offline for maintenance (that is, suspend a server by entering Maintenance Mode), XenServer automatically migrates the virtual machines running on that host to their optimal servers when available. XenServer migrates them based on Workload Balancing recommendations (performance data, your placement strategy, and performance thresholds).
If an optimal server is not available, the words Click here to suspend the VM
appear in the Enter Maintenance Mode
dialog box. In this case, Workload Balancing does not recommend a placement because no host has sufficient resources to run this virtual machine. You can either suspend this virtual machine or exit Maintenance Mode and suspend a virtual machine on another host in the same pool. Then, if you reenter the Enter Maintenance Mode
dialog box, Workload Balancing might be able to list a host that is a suitable candidate for migration.
Note: When you take a server offline for maintenance and Workload Balancing is enabled, the words "Workload Balancing" appear in the upper-right corner of the Enter Maintenance Mode dialog box.