Re: Contents of "backup_label" and "*.backup" in pg_wal location

On Fri, Nov 4, 2016 at 3:44 PM, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
wrote:

> On Fri, Nov 4, 2016 at 1:18 PM, Venkata B Nagothi <nag1010(at)gmail(dot)com>
> wrote:
> > I see the following contents in the file
> > "000000010000000000000044.00000060.backup" which was generated in the
> pg_wal
> > location during the online backup. When pg_stop_backup() is executed, the
> > following content is written which includes the content copied from the
> > backup_label file.
> >
> > [...]
> >
> > Can someone please help me know the importance of the above file?
>
> It is not actually critical, and useful for debugging (you could say
> the same about backup_label.old).
>
> > How about having the same contents in the backup_label file as well?
>
> > As of now, the backup_label file does not have any information related to
> > when and at what position the backup actually completed.
>
> Yes, and it is not actually possible to write the stop information
> because when a backup finishes the backup_label is simply removed and
> it has been included in the backup before it finishes. The role of
> this file is to provide the LSN start location from which the backup
> is able to replay things cleanly. The stop position, aka when
> everything on disk is consistent, is determined at replay by the
> XLOG_BACKUP_END record. This stop position is not something you can
> know when the backup_label file is generated. And I am of course
> talking about exclusive backups here.
>

Sure. I will look at the possibility of using XLOG_BACKUP_END in my patch.
I am looking at the possibility of keeping the backup_label at source until
pg_stop_backup()
is executed and then write the STOP information and then move it across to
the backup location.

I see that when the START/STOP information is written to the WAL history
file,
the content from the backup_label file is being copied and I am thinking to
do the same other way around.

Am i making sense ? is that anyway not possible ?

If this makes sense, then i would start working on an optimal design and
look at the possibility of achieving this.

Regards,

Venkata B N
Database Consultant

Fujitsu Australia