<off-topic> Porting filesystems to different OS </off-topic>

Filesystems represent an on-disk format. And filesystem code on how to handle the disk depends on the on-disk format, as well as, how the OS deals with filesystems i.e., the VFS, the page-cache techniques, write-back strategy, memory management etc.. Unless all OSes(unices?) are almost the same, using the same code across OSes with minimal changes may not be optimal?