Re: Allowing WAL fsync to be done via O_SYNC

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> 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.

I believe that we don't know enough yet to nail down a hard-wired
decision.  Vadim's idea of preferring O_DSYNC if it appears to be
different from O_SYNC is a good first cut, but I think we'd better make
it possible to override that, at least for testing purposes.

So I think it should be configurable at *some* level.  I don't much care
whether it's a config.h entry or a GUC variable.

But consider this: we'll be more likely to get some feedback from the
field (allowing us to refine the policy in future releases) if it is a
GUC variable.  Not many people will build two versions of the software,
but people might take the trouble to play with a run-time configuration
setting.

			regards, tom lane