Re: increasing the default WAL segment size

On 2016-08-25 00:28:58 -0400, Robert Haas wrote:
> On Wed, Aug 24, 2016 at 11:52 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2016-08-24 23:26:51 -0400, Robert Haas wrote:
> >> On Wed, Aug 24, 2016 at 10:54 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >> > and I'm also rather doubtful that it's actually without overhead.
> >>
> >> Really? Where do you think the overhead would come from?
> >
> > ATM we do a math involving XLOG_BLCKSZ in a bunch of places (including
> > doing a lot of %). Some of that happens with exclusive lwlocks held, and
> > some even with a spinlock held IIRC. Making that variable won't be
> > free. Whether it's actually measurabel - hard to say. I do remember
> > Heikki fighting hard to simplify some parts of the critical code during
> > xlog scalability stuff, and that that even involved moving minor amounts
> > of math out of critical sections.
>
> OK, that's helpful context.
>
> >> What sort of test would you run to try to detect it?
> >
> > Xlog scalability tests (parallel copy, parallel inserts...), and
> > decoding speed (pg_xlogdump --stats?)
>
> Thanks; that's helpful, too.

FWIW, I'm also doubtful that investing time into making this initdb
configurable is a good use of time: The number of users that'll adjust
initdb time parameters is going to be fairly small.