It would be nice to have preallocated database files be more configurable than "on" and "off". In our environment, we create a database per customer. For each customer that does nothing, we still write a little bit of data to the database, therefore triggering preallocation. So, each idle customer's preallocated file size on disk is 3x what it needs to be.

I know of the noprealloc flag. I would prefer something like preallocat=80%, or 50%, or whatever. That is, a way to tell mongo "don't preallocate a new file until you're actually close to needing it, and here's how close you should be". I recognize the benefit of preallocation, and would like to keep the advantages, without the disadvantage of preallocating files that will never be used.