postcopying

Postcopying is a memory migration technique used in virtual machine live migration. During a live migration, the goal is to move a running virtual machine from one physical host to another with minimal interruption to its services. Traditional pre-copying methods involve transferring the entire memory of the virtual machine before it can be switched over. Postcopying takes a different approach.

In postcopying, the virtual machine is initially switched over to the destination host with only a minimal amount of its memory. The remaining memory pages are transferred as needed, on demand, after the virtual machine has started running on the new host. When the virtual machine on the destination host attempts to access a memory page that has not yet been transferred, a page fault occurs. This page fault triggers the transfer of that specific required page from the source host to the destination.

This on-demand transfer allows the migration to complete much faster in scenarios where the virtual machine's memory is large or changes rapidly. The downside is that if the virtual machine accesses a significant amount of its memory that hasn't been migrated, the migration process can be slower and more disruptive due to repeated page faults and transfers. Postcopying is often used in conjunction with pre-copying to achieve a balance between migration speed and performance during the migration process.