Bruce Momjian <pgman@candle.pha.pa.us> writes:
> So it does I/O directly from the user buffer to disk, bypassing the
> system cache. I am not sure if that is a good idea because you are not
> using the system buffer cache nor is it allowing writes to be re-ordered
> for optimial performance.
... and, more than likely, the user program is blocked for the whole
physical write operation, not just for long enough to memcpy the data
into a kernel buffer. Given that info, I find it completely
unsurprising that this "feature" makes Postgres a lot slower. It seems
that Sun's idea of what a database does has little connection to what
Postgres does.
It might possibly make sense to set this bit on WAL writes, but not on
writes to data files.
regards, tom lane