+ # Retrieve cache age histogram info from logs.
+
+ # In the logs we expect to find it in an ordered list with entries
+ # of the form (mtime, disk proportion free).
+
+ # An entry of the form (1388747781, 0.52) means that if we deleted
+ # the oldest non-presisted blocks until we had 52% of the disk
+ # free, then all blocks with an mtime greater than 1388747781
+ # would be preserved.
+
+ # The chart we want to produce, will tell us how much of the disk
+ # will be free if we use a cache age of x days. Therefore we will
+ # produce output specifying the age, cache and persisted. age is
+ # specified in milliseconds. cache is the size of the cache if we
+ # delete all blocks older than age. persistent is the size of the
+ # persisted blocks. It is constant regardless of age, but it lets
+ # us show a stacked graph.
+
+ # Finally each entry in cache_age_histogram is a dictionary,
+ # because that's what our charting package wats.