Re: [HACKERS] Reading backup label file for checkpoint and redolocation during crash recovery

Thank you! Got it.

-----Original Message-----
From: Stephen Frost [mailto:sfrost@snowman.net]
Sent: Monday, September 25, 2017 10:57 AM
To: Magnus Hagander <magnus@hagander.net>
Cc: Satyanarayana Narlapuram <Satyanarayana.Narlapuram@microsoft.com>; PostgreSQL-development
<pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Reading backup label file for checkpoint and redo location during crash recovery

* Magnus Hagander (magnus@hagander.net) wrote:
> On Mon, Sep 25, 2017 at 7:43 PM, Stephen Frost <sfrost@snowman.net> wrote:
> > * Satyanarayana Narlapuram (Satyanarayana.Narlapuram@microsoft.com) wrote:
> > > During crash recovery, last checkpoint record information is
> > > obtained
> > from the backup label if present, instead of getting it from the
> > control file. This behavior is causing PostgreSQL database cluster
> > not to come up until the backup label file is deleted (as the error message says).
> > >
> > > if (checkPoint.redo < checkPointLoc)
> > >                       {
> > >                              if (!ReadRecord(xlogreader,
> > checkPoint.redo, LOG, false))
> > >                                     ereport(FATAL,
> > >                                                   (errmsg("could
> > > not
> > find redo location referenced by checkpoint record"),
> > >                                                   errhint("If you
> > > are
> > not restoring from a backup, try removing the file
> > \"%s/backup_label\".", DataDir)));
> > >                       }
> > >
> > > If we are recovering from a dump file, reading from the backup
> > > label
> > files makes sense as the control file could be archived after a few
> > checkpoints. But this is not the case for crash recovery, and is
> > always safe to read the checkpoint record information from the control file.
> > > Is this behavior kept this way as there is no clear way to
> > > distinguish
> > between the recovery from the dump and the regular crash recovery?
> >
> > This is why the exclusive backup method has been deprecated in PG10
> > in favor of the non-exclusive backup method, which avoids this by
> > not creating a backup label file (it's up to the backup software to
> > store the necessary information and create the file for use during recovery).
>
> Actally, it was deprecated already in 9.6, not just 10.

Whoops, right.  Thanks for the clarification. :)

Stephen


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