Currently, MongoDB allocates the namespace for new collections based on the 'nssize' parameter passed during the initial launch of mongod (or defaults to 16MB). While it's possible to query the mongod for the nssize parameter as passed on the command line, this number doesn't reflect the actual usage of namespaces for existing instances and will in fact differ from the value supplied as the nssize if the collection was created when the nssize parameter was different.

This behavior means it's actually not possible to detect or report the actual allocation for the namespace storage, and thus not possible to detect if the system is about to exceed that allocation (and thus start failing to allocate new names). One must query the operating system to determine the actual allocation.

Mongod should provide a way to query on the size allocated for the namespaces, which would enable features like MMS-787 to provide concrete warnings about potentially exceeding that limit.