title: How to check the available free space in mongodb
body: <p>I am newly starting on mongodb. I know that by default, mongodb pre allocated data files in the order of 64MB, 128MB, 256MB, ....2GB, 2GB, 2GB....etc</p>
  
  <p>Now let us supposed i have a mongo instance that is preallocated with 
  64MB and 128MB for each of the different collections.
  Now I would like to know the below</p>
  
  <ul>
  <li>How to check the available free space in the 128MB file of each collections?</li>
  <li>How to check the total size of all the collections, data bases?</li>
  <li>How to check the total available free space out of allocated?</li>
  </ul>