It looks like doing `sysctl -w kern.always_do_fullfsync=1` Will produce a sane behavior: https://opensource.apple.com/source/xnu/xnu-2422.1.72/bsd/hfs/hfs_vnop

It looks like doing `sysctl -w kern.always_do_fullfsync=1` Will produce a sane behavior: https://opensource.apple.com/source/xnu/xnu-2422.1.72/bsd/hfs/hfs_vnops.c https://opensource.apple.com/source/xnu/xnu-2422.1.72/bsd/hf... Alternatively, you could just use the ZFS driver for Mac OS X. It will do fsync properly.