to not forget the thought...can we read from non-private mmap sometimes, while others are writing to the private view?

(1) this might be clean on secondaries, as the semantics are eventually consistent reads. requiresi research though, in particular with respect to write concern acknowledgements.

(2) perhaps on a primary it makes sense if a group commit is underway and we are in 'W', we could still query during most phases i.e., everything except during WRITETODATAFILES phase).

(3) perhaps while building an index, others can still do queries with the non-private mmaps. index builds span group commits, but that would be ok if the indication of the index's existance is written in the final commit.