We can add two columns for wiredTiger cache which tracks:

• % dirty which will fluctuate with the workload
• % used should just grow til max is hit

Use the following stats:

• db.serverStatus().wiredTiger.cache["tracked dirty bytes in the cache"]
• db.serverStatus().wiredTiger.cache["bytes currently in the cache"]
• db.serverStatus().wiredTiger.cache["maximum bytes configured"]