Re: Allowing WAL fsync to be done via O_SYNC

> > I've been mentally working through the code, and see only one reason why
> > it might be necessary to go with a compile-time choice: suppose we see
> > that none of O_DSYNC, O_SYNC, O_FSYNC, [others] are defined?  With the
> > compile-time choice it's easy: #define USE_FSYNC_FOR_WAL, and sail on.
> > If it's a GUC variable then we need a way to prevent the GUC option from
> > becoming unset (which would disable the fsync() calls, leaving nothing
> > to replace 'em).  Doable, perhaps, but seems kind of ugly ... any
> > thoughts about that?
> 
> I don't think having something a run-time option is always a good idea. 
> Giving people too many choices is often confusing.  
> 
> I think we should just check at compile time, and choose O_* if we have
> it, and if not, use fsync().  No one will ever do the proper timing
> tests to know which is better except us.  Also, it seems O_* should be
> faster because you are fsync'ing the buffer you just wrote, so there is
> no looking around for dirty buffers like fsync().

I later read Vadim's comment that fsync() of two blocks may be faster
than two O_* writes, so I am now confused about the proper solution. 
However, I think we need to pick one and make it invisible to the user. 
Perhaps a compiler/config.h flag for testing would be a good solution.

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