How to store real-time data in limited-sized file in chronological priority?

Planning to use a file with restricted-size to store my real time data in android.

Requirement is:

1) Strategy requires me to prioritise the newer data-lines to the old data-lines. It's okay to lose older data.

2) Write should be fastest (in engineering context).

3) While replacing data-lines, the old-line must not be cleared incompletely.

Planning to use MappedByteBuffer to make write faster.

In case i go for rotate-based update, not sure how to remember it's last edited-position without much overhead.