Unix file open supports a number of options, e.g. O_DIRECT O_NOATIME etc.
Currently open_write takes one option "overwrite", but there are several other options
which would be nice, including CLOSE_COMPLETE (finalize the object when nbytes == ndone
and expect a close, preventing the head from being split from the data in cases where
the object is small) and SYNC which would call COMPLETE when the object has hit
the disk and would be recovered in the case of a crash.