How to manage cache in MongoDB WiredTiger?

Question

MongoDB

I use mongo version 3.0.4 wiredtiger engine on Centos 7. Standard settings in mongodb.conf indicating the type of engine. The problem is that the database cache is full. Reloading the database or dropping the system cache through /proc/sys/vm/drop_caches helps. After the drop, the base is restored in 10-15 minutes, which is very critical, loading the screws (hdd) in the raid by 100%. If you overload the database, then for 3 days it works stably without overflow.
There is no documentation on how to manage the cache, and it is not clear why the database itself does not flush unnecessary pages.
I tried to limit the cache via --wiredTigerCacheSizeGB, mongostat gives 100% database load and the database hangs, eats 40GB out of 64GB by default and hangs in the same way. Please tell me where to dig if anyone faced this problem.
P.S. on version 2.6 and the standard engine, such problems were not observed.

Answer 1

Transferred the base to a sideboard with 128GB of RAM and gave out 80GB for the cache. 2 weeks flight is normal. Updated base to 3.0.6.

Answer 2

Try a fresher version, like 3.1.7 is already there.