Re: Updated backup APIs for non-exclusive backups

From: David Steele <david(at)pgmasters(dot)net>
To: Magnus Hagander <magnus(at)hagander(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Updated backup APIs for non-exclusive backups
Date: 2016-02-10 15:38:42
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On 2/10/16 7:46 AM, Magnus Hagander wrote:
> Per discussion at the developer meeting in Brussels, here's a patch that
> makes some updates to the backup APIs, to support non-exclusive backups
> without using pg_basebackup. <...>

This sounds like a great idea and I have signed up to review.

> * A new version of pg_stop_backup is created, taking an argument
> specifying if it's exclusive or not. The current version of
> pg_stop_backup() continues to work for exclusive backups, with no
> changes to behavior. The new pg_stop_backup will return a record of
> three columns instead of just the value -- the LSN (pglsn), the backup
> label file (text) and the tablespace map file (text). If used to cancel
> an exclusive backup, backup label file and tablespace map will be NULL.
> At this point it's up to the backup software to write down the files in
> the location of the backup.

It would be nice if this new pg_stop_backup() function also output the
exact time when the backup stopped. Depending on how long
pg_stop_backup() waits for WAL to be archived a time-stamp recorded
after pg_stop_backup() returns can be pretty far off.

This information is handy for automating selection of a backup when
doing time-based PITR (or so the user can manually select). For
exclusive backups it is possible to parse the .backup file to get this
information but non-exclusive backups do not output the .backup file.

I would be happy to see the time-stamp returned from the
pg_start_backup() function as well. It's a bigger change, but once
pg_start_backup() returns multiple columns it will be easier to add more
columns in the future.

In fact, it might be better if backup_label and tablespace_map were
output by pg_start_backup(). This would give the backup software more
information to work with from the start.


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