| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: increasing the default WAL segment size |
| Date: | 2016-08-25 03:52:20 |
| Message-ID: | 20160825035220.bsbbbao3btsreov5@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
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.
> 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?)
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