Filesystem Direct I/O and WAL sync option

All,

I'm very curious to know if we may expect or guarantee any data
consistency with WAL sync=OFF but using file system mounted in Direct
I/O mode (means every write() system call called by PG really writes
to disk before return)...

So may we expect data consistency:
   - none?
   - per checkpoint basis?
   - full?...

Thanks a lot for any info!

Rgds,
-Dimitri