Migrate for Compute Engine distinguishes between running a VM in Google Cloud and migrating the VM to Google Cloud.
When running an on-premises VM in Google Cloud, storage is streamed, and storage blocks are only cached on Google Cloud when needed. This dramatically reduces the time to start up a VM on Google Cloud.
Migrating an on-premises VM to Google Cloud involves running the VM in Google Cloud while actively migrating its data in the background to Google Cloud. When the data is fully cached on Google Cloud, the VM is detached. Detaching a VM moves its storage from the Migrate for Compute Engine cache and creates native disks on Compute Engine. When detaching, the VM needs to be shut down briefly to switch from the streaming to native disk volumes and to allow for complete synchronization. You can initiate storage migration at any point in time.
For more information, see VM Migration Lifecycle.