My server configure:
E5 24cores/512GB memory,300GB for wiredTiger/1.5T SSD/linux as6u3 redhat.
My mongod instance configure:
wiredTiger/journal/commit interval:10s/wiredTiger cache 300GB/eviction thread 4-12

I have at most 12 mongoimport process running with only one mongod instance at the same time to load data.I hava about 50 collections.And each collection has about 10 indexes.
Usually,after one or two days,mongoDB hangup.Before mongoDB hangup,it can store almost 1TB data each day.I get that through using show dbs.
I find there always be an INSERT operation hangup through using db.currentOp().It may have been active for thousands or more seconds.It locks the DB which it insert into and blocks all operation after it.db.killOp() doesn't work either.It seems like you are creating an index in that DB.show dbs don't respond.Insert or query in that DB doesn't respond either.But,other DBs are OK.And through mongostat,i find mongod is still running,dirty% decreases.
so ,my question:
Does this question is related to index?
How does mongoDB maintain index?
Is there only one thread can maintain index each DB at the same time?
How to avoid this question?