Index compaction not running

I notice my index disk size is keep growing. Fragmentation is at 99.9 %. However, index compaction is not triggered. Any advise?

I have a small bucket. But the index file is already at 3.7GB.

root@ip-172-31-0-242:/opt/couchbase/var/lib/couchbase/data/@2i/rms_rms-requestType-index_8817124799819864044_0.index# ls -ltr
total 3629556
-rw-rw---- 1 couchbase couchbase 3716661248 Aug 6 20:17 data.fdb.1

@m.chu, which couchbase server version are you using?