RE: Disable WAL logging to speed up data loading

Hello, Laurenz

On Thursday, Nov19, 2020 4:50 PM Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> wrote
> On Thu, 2020-11-19 at 05:24 +0000, osumi(dot)takamichi(at)fujitsu(dot)com wrote:
> > > > > ereport(WARNING,
> > > > > (errmsg("WAL was generated with wal_level=minimal, data
> > > > > may be missing"),
> > > > > errhint("This happens if you temporarily set
> > > > > wal_level=minimal without taking a new base backup."))); There's
> > > > > definitely a question about if a WARNING there is really
> > > > > sufficient or not, considering that you could end up with 'logged'
> > > > > tables on the replica that are missing data, but I'm not sure
> > > > > that inventing a new, independent, mechanism for checking WAL
> > > > > level changes makes
> > > > sense.
> > >
> > > I don't know why WARNING was chosen. I think it should be FATAL,
> > > resulting in the standby shutdown, disabling restarting it, and
> > > urging the user to rebuild the standby. (I guess that's
> > > overreaction because the user may not perform operations that lack
> > > WAL while wal_level is minimal.)
> >
> > Yeah, I agree that WARNING is not sufficient.
>
> I missed that this is only a warning when I looked at it before.
> Yes, it should be a fatal error.
>
> I think that there should two patches: one that turns this warning into a
> FATAL and should be backpatched. If you change the test to
>
> ControlFile->wal_level <= WAL_LEVEL_MINIMAL
>
> it will automatically work for your new feature too.
> Then your new wal_level would be a second patch only for HEAD.
Yeah, this suggestion to divide the patch into two sounds really good.
Thank you. I'll post separated patches next time.

>
> With that, the only remaining consideration with this patch is the danger that
> enabling wal_level=none without taking a backup before can lead to data loss.
> But that is intended, so I think that an unmistakable warning in the
> documentation would be good enough.
Yes, thank you for reviewing the documents in the patch.

Best,
Takamichi Osumi