Re: Remove Deprecated Exclusive Backup Mode

Fujii Masao wrote:
> So, let me clarify the situations;
> 
> (3) If backup_label.pending exists but recovery.signal doesn't, the server
>        ignores (or removes) backup_label.pending and do the recovery
>        starting the pg_control's REDO location. This case can happen if
>        the server crashes while an exclusive backup is in progress.
>        So crash-in-the-middle-of-backup doesn't prevent the recovery from
>        starting in this idea

That's where I see the problem with your idea.

It is fairly easy for someone to restore a backup and then just start
the server without first creating "recovery.signal", and that would
lead to data corruption.

Indeed, if I didn't know much about databases and were faced with
the task of restoring a PostgreSQL database from backup in a hurry,
that is probably what I would do.

Having thought some about the whole problem area, I believe that there
is no safe way to disambiguate a backup from a server crashed in backup
mode.

The non-exclusive backup mode actually suffers from a similar problem ---
if you don't add "backup_label" to the backup afterwards, it will lead
to database corruption.

The only fool-proof tool we have in core is pg_basebackup.

Any really good backup solution that wishes to integrate with a third-
party backup tool will have to get that tool to include "backup_label"
with the backup itself.

For that, it would need to know the contents of "backup_label" *before*
pg_stop_backup().  Is there a good reason why it is pg_stop_backup()
and not pg_start_backup() that provides that information?

Yours,
Laurenz Albe