How to check the pre-allocated database file of MongoDB is Used or not?
I am not very familiar with MongoDB, but from this link:


  As soon as a datafile starts to be used, the next one will be preallocated.


It seems to me that (on *nix at least), MongoDB will always have a datafile that is empty, or mostly empty. If it's writing heavy and filling up the file, the last file might have some data in it before a new empty file can be created.


  Additionally, on Unix, mongod will preallocate an additional datafile in the background and do background initialization of this file. These files are prefilled with zero bytes.


So to determine if it's used, check the file for non-zero bytes.