Quoting from the OpenVZ official annoncement:
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The OpenVZ project announced it will add the industry-exclusive “zero downtime migration” feature that allows IT professionals to move virtual servers between physical servers without end-user disruption or the need for costly storage capacity.Using the zero downtime migration function captures the state of an existing virtual environment and its contents and migrates it to a new physical server without any interruption in service or availability. The function executes between any two servers on a network, so the capability works for any server and any application. OpenVZ delivers this capability without additional requirements, such as a storage area network (SAN). The zero downtime migration feature will be made available for download later this month…