Dear Jonka,
V is meant to be the total data transfer volume, i.e., V = V_r + V_w, where V_r is the read data volume and V_w is the written data volume. The implicit assumption here is that the read and write data streams share the memory interface in a "half-duplex" kind of way, i.e., at any point in time, data is either read or written, but always at full bandwidth. This is how most (but not all) contemporary processors work.
Hope this helps,
Georg.