Re: question: data file update when pg_basebackup in progress

On Wed, Apr 26, 2017 at 1:45 AM, David G. Johnston
<david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
> The first write to a page after a checkpoint is always recorded in the WAL
> as a full page write. Every WAL file since the checkpoint must also be
> copied to the backed up system. The replay of those WAL files is what
> brings the remote and local system into sync with respect to all changes
> since the backup checkpoint.

Bringing to the point that the presence of backup_label in a backup is
critical, as this tells Postgres from which position in WAL it should
begin recovery to bring the system up to a consistent state.
pg_basebackup also makes sure that the last WAL segment needed is
archived before the backup completes so as recovery can completely be
done.
--
Michael