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From: Chapman Flack <[email protected]>
To: Nathan Bossart <[email protected]>
To: David Steele <[email protected]>
Cc: Stephen Frost <[email protected]>
Cc: [email protected]
Subject: Re: Postgres restart in the middle of exclusive backup and the presence of backup_label file
Date: Tue, 8 Mar 2022 20:24:13 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <20220308221253.GA169440@nathanxps13>
References: <20220301173208.GB1031413@nathanxps13>
	<[email protected]>
	<[email protected]>
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	<[email protected]>
	<[email protected]>
	<20220302010302.GA1182956@nathanxps13>
	<[email protected]>
	<20220308200107.GA85558@nathanxps13>
	<[email protected]>
	<20220308221253.GA169440@nathanxps13>

On 03/08/22 17:12, Nathan Bossart wrote:
> I spent some time trying to come up with a workable script to replace the
> existing one.  I think the main problem is that you need to write out both
> the backup label file and the tablespace map file, but I didn't find an
> easy way to write the different output columns of pg_backup_stop() to
> separate files via psql.

Something like this might work:

SELECT * FROM pg_backup_stop(true) \gset

\out /tmp/backup_label   \qecho :labelfile
\out /tmp/tablespace_map \qecho :spcmapfile
\out
\! ... tar command adding /tmp/{backup_label,tablespace_map} to the tarball

I notice the \qecho adds a final newline (and so if :spcmapfile is empty,
a file containing a single newline is made). In a quick test with a bogus
restore_command, I did not see any error messages specific to the format
of the backup_label or tablespace_map files, so maybe the final newline
isn't a problem.

Assuming the newline isn't a problem, that might be simple enough to
use in an example, and maybe it's not a bad thing that it highlights a few
psql capabilities the reader might not have stumbled on before. Or, maybe
it is just too confusing to bother.

While agreeing that pg_basebackup is the production-ready thing that
does it all for you (with tests for likely errors and so on), I think
there is also some value in a dead-simple example that concretely
shows you what "it" is, what the basic steps are that happen beneath
pg_basebackup's chrome.

If the added newline is a problem, I haven't thought of a way to exclude
it that doesn't take the example out of the realm of dead-simple.

Regards,
-Chap

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