Re: O_DSYNC broken on MacOS X?

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

--
Greg Smith, 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services and Support www.2ndQuadrant.us
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