Re: Migration study, step 1: bulk write performance

At 03:44 PM 3/21/2006, Simon Riggs wrote:
>On Mon, 2006-03-20 at 15:59 +0100, Mikael Carneholm wrote:
>
> > This gives that 10Gb takes ~380s => ~27Mb/s (with fsync=off),
> compared to the raw dd result (~75.5Mb/s).
> >
> > I assume this difference is due to:
> > - simultaneous WAL write activity (assumed: for each byte written
> to the table, at least one byte is also written to WAL, in effect:
> 10Gb data inserted in the table equals 20Gb written to disk)
> > - lousy test method (it is done using a function => the
> transaction size is 10Gb, and 10Gb will *not* fit in wal_buffers :) )
> > - poor config
>
> > checkpoint_segments = 3
>
>With those settings, you'll be checkpointing every 48 Mb, which will be
>every about once per second. Since the checkpoint will take a reasonable
>amount of time, even with fsync off, you'll be spending most of your
>time checkpointing. bgwriter will just be slowing you down too because
>you'll always have more clean buffers than you can use, since you have
>132MB of shared_buffers, yet flushing all of them every checkpoint.
IIRC, Josh Berkus did some benches that suggests in pg 8.x a value of
64 - 256 is best for checkpoint_segments as long as you have the RAM available.

I'd suggest trying values of 64, 128, and 256 and setting
checkpoint_segments to the best of those.

Ron