$showDiskLoc option adds a field $diskLoc to the returned documents. The $diskLoc field contains the disk location information.
The mongo shell provides the cursor.showDiskLoc() method:
db.collection.find().showDiskLoc()
You can also specify the option in either of the following forms:
db.collection.find( { <query> } )._addSpecial("$showDiskLoc" , true)
db.collection.find( { $query: { <query> }, $showDiskLoc: true } )