Keep your disks when a VM is deleted.
gcloud compute instances set-disk-auto-delete server --no-auto-delete --disk=server-disk-1 gcloud compute instances set-disk-auto-delete server --no-auto-delete --disk=server-disk-2 |
To delete.
gcloud compute instances set-disk-auto-delete server --auto-delete --disk=server-disk-1 gcloud compute instances set-disk-auto-delete server --auto-delete --disk=server-disk-2 |