"Drivers have been made to behave when using a bigger than 128KB maximum IO transfer size"

[Wojciech Puchar]

How to turn in on in dragonfly? i mean setting maximum disk I/O to 2 or 
4MB - same as i do in FreeBSD to get large performance gain on big files.

in FreeBSD
options         MAXPHYS=2097152 (or twice that)
options         DFLTPHYS=262144