This is the number of bytes to be moved in the data transfer associated with the command. The direction of the transfer is indicated by 'dxfer_direction'. If 'dxfer_len' is zero then no data transfer takes place.
If iovec_count is non-zero then 'dxfer_len' should be equal to the sum of iov_len lengths. If not, the minimum of the 2 is the transfer length. The type of dxfer_len is unsigned int.