Re: how to demonstrate the effect of direct I/O ?

On 5.2.2012 00:25, Tomas Vondra wrote:
> Hi all,
>
> I've been running a lot of benchmarks recently (I'll publish the results
> once I properly analyze them). One thing I'd like to demonstrate is the
> effect of direct I/O when the wal_fsync_method is set to
> open_sync/open_datasync.
>
> I.e. I'd like to see cases when this improves/hurts performance
> (compared to fsync/fdatasync) and if/how this works on SSD compared to
> old-fashioned HDD. But no matter what, I see no significant differences
> in performance.

BTW the benchmark suite I run consists of two parts:

  (a) read-write pgbench
  (b) TPC-H-like benchmark that loads a few GBs of data (and then
      queries them)

I'd expect to see the effect on the TPC-H load part, and maybe on the
pgbench (not sure if positive or negative).

> All this was run on a kernel 3.1.5 using an ext4 filesystem.

And the Pg versions tested were 9.1.2 and the current 9.2dev snapshot.


Tomas