fsync is not enough on OS X

[djs55]

man fsync has:

Note that while fsync() will flush all data from the host to the drive (i.e.
the "permanent storage device"), the drive itself may not physically write the
data to the platters for quite some time and it may be written in an out-of-
order sequence.
?
Specifically, if the drive loses power or the OS crashes, the application may
find that only some or none of their data was written. The disk drive may
also re-order the data so that later writes may be present, while earlier
writes are not.

This is not a theoretical edge case. This scenario is easily reproduced with
real world workloads and drive power failures.

For applications that require tighter guarantees about the integrity of their
data, Mac OS X provides the F_FULLFSYNC fcntl. The F_FULLFSYNC fcntl asks the
drive to flush all buffered data to permanent storage. Applications, such as
databases, that require a strict ordering of writes should use F_FULLFSYNC to
ensure that their data is written in the order they expect. Please see
fcntl(2) for more detail.

man fnctl has:

F_FULLFSYNC Does the same thing as fsync(2) then asks the drive to
flush all buffered data to the permanent storage device
(arg is ignored). This is currently implemented on HFS,
MS-DOS (FAT), and Universal Disk Format (UDF) file systems.
The operation may take quite a while to complete. Certain
FireWire drives have also been known to ignore the request
to flush their buffered data.

hyperkit has ioctl(fd, DKIOCSYNCHRONIZECACHE)

From this book it looks like fcntl(F_FULLFSYNC) flushes journals before ultimately calling ioctl(fd, DKIOCSYNCHRONIZECACHE)