A.M. wrote:
That is not correct. fsync and friends on Darwin synchronizes I/O and flushes dirty kernel caches to the disk which meets the specification and is distinctly different from doing nothing... "On MacOS X, fsync() always has and always will flush all file data from host memory to the drive on which the file resides." http://lists.apple.com/archives/Darwin-dev/2005/Feb/msg00072.html
You didn't quote the next part of that, which says "fsync() is not sufficient to guarantee that your data is on stable storage and on MacOS X we provide a fcntl(), called F_FULLFSYNC, to ask the drive to flush all buffered data to stable storage." That's exactly what turning on fsync_writethrough does in PostgreSQL. See http://archives.postgresql.org/pgsql-hackers/2005-04/msg00390.php as the first post on this topic that ultimately led to that behavior being implemented.
From the perspective of the database, whether or not the behavior is standards compliant isn't the issue. Whether pages make it to physical disk or not when fsync is called, or when O_DSYNC writes are done on platforms that support them, is the important part. If you the OS doesn't do that, it is doing nothing useful from the perspective of the database's expectations. And that's not true on Darwin unless you specify F_FULLFSYNC, which doesn't happen by default in PostgreSQL. It only does that when you switch wal_sync_method=fsync_writethrough
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