Re: Final background writer cleanup for 8.3

"Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov> writes:

> Is it my imagination, or are we coming pretty close to the point where we
> could accomadate the oft-requested feature of dealing directly with a raw
> volume, rather than going through the file system at all?

Or O_DIRECT.

I think the answer is that we've built enough intelligence that it's feasible
from the memory management side.

However there's another side to that problem. a) you would either need to have
multiple bgwriters or have bgwriter use aio since having only one would
serialize your i/o which would be a big hit to i/o bandwidth. b) you need some
solution to handle preemptively reading ahead for sequential reads.

I don't think we're terribly far off from being able to do it. The traditional
response has always been that our time is better spent doing database stuff
rather than reimplementing what the OS people are doing better. And also that
the OS has more information about the hardware and so can schedule I/O more
efficiently.

However there's also a strong counter-argument that we have more information
about what we're intending to use the data for and how urgent any given i/o is
so.

I'm not sure how that balancing act ends. I have a hunch but I guess it would
take experiments to get a real answer. And the answer might be very different
on different OSes and hardware configurations.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com