How to do it...

1. Start the mongod daemon to explicitly use MMAPv1 storage engine:

/data/mongodb/bin/mongod --dbpath /data/db --storageEngine mmapv1

1. Start the mongo client and you should be presented with the MongoDB shell. Execute the following commands in the shell:

> var status = db.serverStatus()
> status['storageEngine']
{
   "name" : "mmapv1",
   "supportsCommittedReads" : false,
   "readOnly" : false,
   "persistent" : true
}

1. Now let's add some random data into it. Run the following JavaScript code to insert 100 documents with random data:

> use mydb
> for(var x=0; x<100; x++){
  db.mycol.insert({
     age:(Math.round(Math.random()*100)%20)
     })
  }
> db.mycol.count()
100

1. Exit the shell and perform a full backup using mongodump command:

mkdir /data/backup
mongodump -o /data/backup --host localhost:27017

1. Now shutdown the mongod process.
2. Create a new data directory for the migration and start the mongod daemon with a new storage engine:

mkdir /data/newdb
/data/mongodb/bin/mongod --dbpath /data/newdb --storageEngine wiredTiger

1. Let's restore the previous backup to this new instance:

mongorestore /data/backup/

1. Start the mongo shell and check your data:

> var status = db.serverStatus()
> status['storageEngine']
{
    "name" : "wiredTiger",
    "supportsCommittedReads" : true,
    "readOnly" : false,
    "persistent" : true
}
> use mydb
switched to db mydb
> db.mycol.count()
100