+text "
+dss snapshot aging is implemented in terms of intervals. There are
+two command line options related to intervals: the duration of a
+'unit' interval and the number of those unit intervals.
+
+dss removes any snapshots older than the given number of intervals
+times the duration of an unit interval and tries to keep the following
+number of snapshots per interval:
+
+ interval number number of snapshots
+ ===============================================
+ 0 2 ^ (num_intervals - 1)
+ 1 2 ^ (num_intervals - 2)
+ 2 2 ^ (num_intervals - 3)
+ ...
+ num_intervals - 2 2
+ num_intervals - 1 1
+ num_intervals 0
+
+In other words, the oldest snapshot will at most be unit_interval *
+num_intervala old (= 5 days * 4 = 20 days if default values are used).
+Moreover, there are at most 2^num_intervals - 1 snapshots in total
+(i.e. 31 by default). Observe that you have to create at least
+num_intervals snapshots each interval for this to work out. "