DTCS uses min_threshold to decide how many time windows of the same size that need to accumulate before merging into a larger window. The age of an SSTable is determined as its min timestamp, and it always falls into exactly one of the time windows. If multiple SSTables fall into the same window, DTCS considers compacting them, but if they are fewer than min_threshold, it decides not to do it.
When do more than 1 but fewer than min_threshold SSTables end up in the same time window (except for the current window), you might ask? In the current state, DTCS can spill some extra SSTables into bigger windows when the previous window wasn't fully compacted, which happens all the time when the latest window stops being the current one. Also, repairs and hints can put new SSTables in old windows.
I think, and Jeremiah Jordan agreed in a comment on
CASSANDRA-6602, that DTCS should ignore min_threshold and compact tables in the same windows regardless of how few they are. I guess max_threshold should still be respected.
Jeremiah Jordan suggested that this should apply to all windows but the current window, where all the new SSTables end up. That could make sense. I'm not clear on whether compacting many SSTables at once is more cost efficient or not, when it comes to the very newest and smallest SSTables. Maybe compacting as soon as 2 SSTables are seen is fine if the initial window size is small enough? I guess the opposite could be the case too; that the very newest SSTables should be compacted very many at a time?