Re: Reading backup label file for checkpoint and redo location during crash recovery

* Magnus Hagander (magnus(at)hagander(dot)net) wrote:
> On Mon, Sep 25, 2017 at 7:43 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
> > * Satyanarayana Narlapuram (Satyanarayana(dot)Narlapuram(at)microsoft(dot)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