Tradeoff of write-ability and erase-ability in overwrite

In magnetic recording, overwrite (OVW) measures how well the old data is suppressed by the new data [1]. In perpendicular recording, OVW is typically gauged by overwriting a high-frequency data pattern (e.g. 2T) with a low-frequency data pattern (e.g. 13T), and the OVW is called Reverse OVW (ROVW) [2]. It is frequently observed that higher OVW doesn't lead to higher Signal to Noise Ratio (SNR) or better channel Error Margin (EM) as Fig. 1 shows, especially when there is significant difference between head/medium designs (D1 and D2); OVW might even be higher at lower write current, which is contradicted to the common observations. It is always a question on how much OVW is good enough, and what does OVW really mean.