To: pgsql-performance@postgresql.org
From: Ivan Voras <ivoras@freebsd.org>
Subject: Re: Backup strategies
Date: Wed, 15 Oct 2008 16:05:06 +0200
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Message-ID: <gd4tad$qlg$1@ger.gmane.org>
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	<alpine.DEB.1.10.0810151432280.15851@aragorn.flymine.org>
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Matthew Wakeling wrote:
> On Wed, 15 Oct 2008, Ivan Voras wrote:
>>> Nope. Even files in data directory change. That's why the documentati=
on
>>> warns against tools that emit errors for files that change during the=

>>> copy.
>>
>> Ok, thanks. This is a bit off-topic, but if it's not how I imagine it,=

>> then how is it implemented?
>=20
> The files may change, but it doesn't matter, because there is enough
> information in the xlog to correct it all.

I'm thinking about these paragraphs in the documentation:

"""
Be certain that your backup dump includes all of the files underneath
the database cluster directory (e.g., /usr/local/pgsql/data). If you are
using tablespaces that do not reside underneath this directory, be
careful to include them as well (and be sure that your backup dump
archives symbolic links as links, otherwise the restore will mess up
your tablespaces).

You can, however, omit from the backup dump the files within the
pg_xlog/ subdirectory of the cluster directory. This slight complication
is worthwhile because it reduces the risk of mistakes when restoring.
This is easy to arrange if pg_xlog/ is a symbolic link pointing to
someplace outside the cluster directory, which is a common setup anyway
for performance reasons.
"""

So, pg_start_backup() freezes the data at the time it's called but still
data and xlog are changed, in a different way that's safe to backup? Why
not run with pg_start_backup() always enabled?


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