Assume you have metric M with 'Insert 0s for missing values' = YES. Assume, its last point is 2016-07-01, and you are going to recollect data from 2016-01-01.

Zeroed values are generated for interval between last point and now (i.e., from 2016-07-01 to 2016-08-05 in the example). If the incoming dataset has holes between 2016-07-01 and 2016-08-05, then these holes will be filled with 0s.

The system will not insert zeros for holes that occurred before the last measurement time. If you want the system to to fill such holes, you will need to delete historical data first, thereby forcing the system to reset the last measurement time.

The same algorithm works for segmented metrics as well. Each segment value is considered separately (last point for particular segment value is used).

Algorithm is limited by 10K points for single generation. If last point for daily metric is 1900-01-01, then interval will be from 1900-01-01 till 1927-05-20.