How to troubleshoot error "idx=... Throttling indexer, too many tsidx files in bucket=..." on my distributed management console?

Hi, I noticed this popping up on my Distributed Management Console the other day: throttled - idx=ngccc_app_logs Throttling indexer, too many tsidx files in bucket='/apps/splunk/db/ngccc_app_logs/db/hot_v1_20299'. Is splunk-optimize working? If not, low disk space may be the cause. My systems have lots of space (2+tb) available. Never seen this one before - any suggestions? It's a high volume index.