perform (Net::SFTP::Operations::Read)

    # File lib/net/sftp/operations/read.rb, line 34
        def perform( handle, options={} )
          @length = options[:length] || -1
          @handle = handle
          @offset = options[:offset] || 0
          @chunk_size = options[:chunk_size] || DEFAULT_CHUNK_SIZE
          @progress_callback = options[:progress_callback]
          @data = ""

          real_length = ( @length >= 0 && @length < @chunk_size ?
                              @length : @chunk_size )
          @driver.read( nil, @handle, @offset, real_length )
        end