This paper discusses live, in-service OS migration as opposed to migration at the process-level. Reasons for choosing this granularity include avoiding "residual dependencies" that require original host machines to remain accessible to migrated processes, transferring in-memory state consistently and efficiently, and separating the concerns of users and operators.
The approach to moving memory used is pre-copy migration, a multi-step process combining push and stop-and-copy techniques. The first phase in a push phase in which all pages are transferred to the new VM, and future phases transfer only pages that were modified during the previous round. Once the remaining pages are part of the frequently-written writable working set, the current VM is suspended, the remaining pages are transferred, and the new VM is started. This pre-copy approach improved upon the amount of downtime caused by a full stop-and-copy significantly for a number of different migrations.
The authors ignore the issue of having to deal with local resources; they make the assumption that migrations tend to happen in a cluster environment within a single LAN and that data centers tend to use network-attached storage in place of local disks.
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