We upgraded our sharded cluster from mongo 3.6 to 4.0.18 on Linux Stretch. Since then IO utilization is almost always at 100%Unable to render embedded object: File ( Almost all monitored metrics look worse than before the upgrade) not found. I'll attach a screenshot where you can see that IO utilization went up to almost constant 100% at around 0:00 o'clock when we upgraded the shard to mongodb 4.0.18 

We did not yet set FCV to 4.0 to allow our deployment to run without enabling these features for a burn-in period to ensure the likelihood of downgrade is minimal - as suggested in mongodb's documentation.

Any help is greatly appreciated because this issue becomes critical!