RRD

RRD, the round-robin database is a format which stores timeseries data.

It's often used to show recent server status details (load, memory usage, bandwidth usage, etc).

The file is a fixed-size rung-buffer, so it only shows a fixed amount of history.

The magic of this file is "RRD\0".

Links

 * Wikipedia article
 * https://oss.oetiker.ch/rrdtool/