size_gb The size, in GB, of the disk to attach. If the size is not
specified, a default is chosen to ensure reasonable I/O
If the disk type is specified as ``local-ssd``, multiple
local drives are automatically combined to provide the
requested size. Note, however, that each physical SSD is
375GB in size, and no more than 8 drives can be attached to
a single instance.
type_ The Compute Engine disk type. If unspecified,
``pd-standard`` is used.
source_image An image to put on the disk before attaching
it to the VM.