When Migrate for Compute Engine migrates a VM from vSphere, all changes on the disk attached to the Google Cloud instance persist back to the source disk. For migrations from AWS, writing changes back to the source is not possible.
Write back mode (default)
In write back mode, Migrate for Compute Engine writes all disk changes made to the VM on Google Cloud back on-premises. Changes are buffered and synchronized once per hour or after 5 GB of changes (whichever comes first).
When you move a VM with this option enabled back to vSphere, Migrate for Compute Engine writes any data remaining in the buffer before restarting the VM. When Migrate for Compute Engine completes the move back, it deletes all interim snapshots and virtual disks.
Write isolation mode
In write isolation mode, Migrate for Compute Engine does not write storage data on-premises while the VM is running on Google Cloud.
Changing storage policy
You can change a VM's storage policy from vSphere by right-clicking the VM and selecting Reconfigure virtual machine.