mongodb-cache-usage.js

let getCache = () => {
  let maxCache = db.serverStatus().wiredTiger.cache['maximum bytes configured']
  let useCache = db.serverStatus().wiredTiger.cache['bytes currently in the cache']
  return {
    maxCache: maxCache / 1024 / 1024, // Convert to MB
    useCache: useCache / 1024 / 1024, // Convert to MB
    usePercent: (useCache / maxCache  * 100).toFixed(2) + '%', // Percentage of cache used
  }
}

let interval = setInterval(() => console.log(getCache()), 1000)