Aggregates are DTrace’s way of accumulating a lot of individual pieces of data into a more useful overall view. Seeing individual read() times can be useful, if a bit voluminous. Having a min / max / average of read times may be much more interesting.
The syntax for using a DTrace aggregate looks a bit like using array.
@name[key] = aggfunc();
The leading at-sign means that an aggregate is being used, and the assignment is a function rather than an expression. There are a number of aggregating functions:
Keeps count of the number of times it is called.
Accumulates the total value of the expression over time.
Accumulates the arithmetic average of the expression over time. ...