Re: [PATCHES] O_DIRECT for WAL writes

Tom Lane wrote:
> Greg Stark <gsstark@mit.edu> writes:
> > Tom Lane <tgl@sss.pgh.pa.us> writes:
> >> Unfortunately, I cannot believe these numbers --- the near equality of
> >> fsync off and fsync on means there is something very wrong with the
> >> measurements.  What I suspect is that your ATA drives are doing write
> >> caching and thus the "fsyncs" are not really waiting for I/O at all.
> 
> > I wonder whether it would make sense to have an automatic test for this
> > problem. I suspect there are lots of installations out there whose admins
> > don't realize that their hardware is doing this to them.
> 
> Not sure about "automatic", but a simple little test program to measure
> the speed of rewriting/fsyncing a small test file would surely be a nice
> thing to have.
> 
> The reason I question "automatic" is that you really want to test each
> drive being used, if the system has more than one; but Postgres has no
> idea what the actual hardware layout is, and so no good way to know what
> needs to be tested.

Some folks have battery-backed cached controllers so they would appear
as not handling fsync when in fact they do.

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