In one embodiment, a system for migrating virtual machines in a virtualization environment includes a plurality of host machines implementing the virtualization environment and a migration controller. Each of the host machines includes a hypervisor, one or more user virtual machines (UVMs) and a virtual machine controller. The system further implements a virtual disk comprising a plurality of storage devices, the virtual disk being accessible by the virtual machine controllers, which conduct I/O transactions with the virtual disk. The migration controller determines a segment size and, for each host machine, determines a number of required segments for the UVMs running on the host machine. The controller computes a number of reserved segments based on a total number of host machines and a largest one of the numbers of required segments. The reserved segments are then assigned among the plurality of host machines.