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

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.

For non-exclusive backups, it could be possible to add some
information related to the backup stop locations directly in the
backup_label file generated and returned to the caller, though the
information it would contain does not seem much pertinent as what
really matters is the LSN end location, and that's already a piece of
information available.
--
Michael