The quick answer, no, I am not aware that OH2 allows persistence strategies with a trigger threshold. Here some other thoughts:
- If you are worried about disk space you could persist through using rrd4j which remains always at a fixed space (by making historical data less granular). Note: rrd4j works only for numbers.
- even if you are persisting every hour it is unlikely that you would run into disk space problems any time soon: 16 data points once an hour makes approx 140.000 data points per year…not really anything big (even every 30mins does not seem excessive).
- You can write rules that iterate over group members and therefore it is likely that you just could write one rule for all your items, this thread will likely give you all you need (once you familiarized yourself with rules in general): Design Pattern: Working with Groups in Rules