On Thu, Jan 16, 2014 at 08:48:24PM -0500, Robert Haas wrote:
> On Thu, Jan 16, 2014 at 7:31 PM, Dave Chinner <david@fromorbit.com> wrote:
> > But there's something here that I'm not getting - you're talking
> > about a data set that you want ot keep cache resident that is at
> > least an order of magnitude larger than the cyclic 5-15 minute WAL
> > dataset that ongoing operations need to manage to avoid IO storms.
> > Where do these temporary files fit into this picture, how fast do
> > they grow and why are do they need to be so large in comparison to
> > the ongoing modifications being made to the database?
[ snip ]
> Temp files are something else again. If PostgreSQL needs to sort a
> small amount of data, like a kilobyte, it'll use quicksort. But if it
> needs to sort a large amount of data, like a terabyte, it'll use a
> merge sort.[1]
IOWs the temp files contain data that requires transformation as
part of a query operation. So, temp file size is bound by the
dataset, growth determined by data retreival and transformation
rate.
IOWs, there are two very different IO and caching requirements in
play here and tuning the kernel for one actively degrades the
performance of the other. Right, got it now.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com