Migrate for Compute Engine distinguishes between running a VM in GCP and migrating the VM to GCP.
When running an on-premises VM in GCP, storage is streamed, and storage blocks are only cached on GCP when needed. This dramatically reduces the time to start up a VM on GCP.
Migrating an on-premises VM to GCP involves running the VM in GCP while actively migrating its data in the background to GCP. When the data is fully cached on GCP, 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.