Re: [PATCHES] O_DIRECT for WAL writes

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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.

It shouldn't be too hard to test a few hundred or even a few thousand fsyncs
and calculate the seek time. If it implies a rotational speed over 15kRPM then
you know the drive is lying and the data storage is unreliable.

--
greg