Datafiles are preallocated on *NIX platforms for optimal performance. However, in some odd use cases where it is desirable to have a lot of databases that are smaller in size interspersed with a few larger DBs, this causes the disk to grow excessively. Given some of the DBs will be split out into multiple 2GB datafiles while the majority will be in 64MB (with 128MB sitting around empty, for perhaps a few kB of data in most), this is a lot of wasted disk space. --noprealloc will impair performance of the larger DBs. Sharding is an option but a waste of hardware.

It would be really nice to be able to pick a smaller starting datafile size. I would think it to be sane to lock it down to a few potential options (starting at 1MB would be great, increasing to 2, 4, 8, ... MB). That would create a lot more data files but modern filesystems are OK with that.

I consider this ticket to be a "metoo" of one of the latter comments in: http://jira.mongodb.org/browse/SERVER-1181