Precopying

Precopying is a technique used in computing to transfer data from one location to another by copying it in advance. This is often done to optimize performance by reducing latency or to ensure data availability.

In the context of virtualization, precopying is a common method used in live migration. When a virtual machine is moved from one host to another without downtime, precopying involves transferring the memory pages of the virtual machine to the destination host while the virtual machine is still running on the source host. This process is iterative, with the source host sending changed memory pages to the destination host until the amount of changed data becomes small enough to transfer quickly. Once the remaining data is sent, the virtual machine's state is frozen, the final changes are transferred, and the virtual machine is resumed on the destination host.

This approach minimizes the downtime experienced by the virtual machine during migration. Without precopying, the entire memory of the virtual machine would need to be transferred while it is stopped, leading to a longer period of unavailability. Precopying helps to reduce the "write penalty" by ensuring that the bulk of the data is already at the destination before the migration is finalized.

The effectiveness of precopying depends on the rate at which memory pages change. If the memory is highly dynamic, precopying might require more iterations and network bandwidth. Techniques like iterative precopying and hybrid approaches that combine precopying with other migration strategies are employed to manage this variability.