Re: 8.4 open item: copy performance regression?

Robert Haas wrote:
> On Sun, Jun 21, 2009 at 11:52 AM, Tom Lane<tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> So to my mind, the only question left to answer (at least for the 8.4
>> cycle) is "is 16MB enough, or do we want to make the ring even bigger?".
>> Right at the moment I'd be satisfied with 16, but I wonder whether there
>> are scenarios where 32MB would show a significant advantage.
>
> Even 32MB is not that much. It seems to me that in any realistic
> production scenario you're going to have at least half a gig of shared
> buffers, so we're really talking about at most one-sixteenth of the
> shared buffer arena, and possibly quite a bit less. I think that's
> pretty conservative.

I was going to say that since we flush the WAL every 16MB anyway (at
every XLOG file switch), you shouldn't see any benefit with larger ring
buffers, since to fill 16MB of data you're not going to write more than
16MB WAL. But then I realized that that's not true if you have an
unusually low fillfactor. If you only fill each page say 50% full,
you're going to use 32MB worth of data pages but only write 16MB of WAL.
And maybe you could have a situation like that with very wide rows as
well, with wasted space on each page that's not enough to store one more
row.

Could you test that scenario?

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com