Re: GUC for data checksums

On 2013-09-16 14:43:27 +0300, Heikki Linnakangas wrote:
> On 15.09.2013 17:05, Andres Freund wrote:
> >On 2013-09-15 03:34:53 +0200, Bernd Helmle wrote:
> >>
> >>
> >>--On 15. September 2013 00:25:34 +0200 Andres Freund
> >><andres(at)2ndquadrant(dot)com> wrote:
> >>
> >>>Looks like a good idea to me. The implementation looks sane as well,
> >>>except that I am not sure if we really need to introduce that faux
> >>>variable. If the variable cannot be set and we have a SHOW hook, do we
> >>>need it?
> >>
> >>It's along the line with the other informational variables like block_size
> >>et al. Do you want to have a function instead or what's your intention?
> >
> >Well, you've added a "data_checksums" variable that won't ever get used,
> >right? You can't set the variable and the show hook doesn't actually use
> >it.
> >The reason you presumably did so is that there is no plain variable that
> >contains information about data checksums, we first need to read the
> >control file to know whether it's enabled and GUCs are initialized way
> >earlier than that.
> >
> >A quick look unfortunately shows that there's no support for GUCs
> >without an actual underlying variable, so unless somebody adds that,
> >there doesn't seem to be much choice.
> >
> >I think a comment documenting that the data_checksums variable is not
> >actually used would be appropriate.
>
> Surprisingly we don't have any other gucs that would be set at initdb time,
> and not changed after that. But we used to have two, lc_collate and
> lc_ctype, until we changed them to be per-database settings. We used to do
> this in ReadControlFile:
>
> > /* Make the fixed locale settings visible as GUC variables, too */
> > SetConfigOption("lc_collate", ControlFile->lc_collate,
> > PGC_INTERNAL, PGC_S_OVERRIDE);
> > SetConfigOption("lc_ctype", ControlFile->lc_ctype,
> > PGC_INTERNAL, PGC_S_OVERRIDE);
>
> I did the same for data_checksums, and committed. Thanks for the patch.

I don't think it's fatal - as you say we've done so before - but note
that both the committed and Bernd's version don't report the correct
value on postgres -C data_checksums because we haven't read the control
file yet...
Maybe we should do that earlier?

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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