Thanks Colin P. McCabe for the review.
This JavaDoc comment is stale. It seems like it should be "the current modification epoch" or similar.
Will update it in the new patch right now.
Can you put this in a separate JIRA? It's less obvious to me that this is a win. Java HashMaps don't ever shrink, whereas TreeMap uses less memory when elements are removed.
Sure, I will put it in a separate JIRA.
That's right HashMap don't ever shrink when elements are removed, but TreeMap entry needs to store more (memory) references (left, right, parent) than HashMap entry (only one reference next), even when there is element removing and cause some entry empty, the empty HashMap entry is just a null reference (4 bytes), so they are close at this point. On the other hand, the key of excessReplicateMap is datanode uuid, so the entries number is almost fixed, so HashMap memory is good than TreeMap memory in this case. I think the most important is the search/insert/remove performance, HashMap is absolutely better than TreeMap. Because we don't need to sort, we should use HashMap instead of TreeMap.