From: Stephen Frost <sfrost@snowman.net>
To: Jeff Janes <jeff.janes@gmail.com>
Cc: julyanto SUTANDANG <julyanto@equnix.co.id>
Cc: Dinesh Chandra 12108 <Dinesh.Chandra@cyient.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Backup taking long time !!!
Date: Mon, 23 Jan 2017 12:23:01 -0500
Message-ID: <20170123172301.GH18360@tamriel.snowman.net> (raw)
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* Jeff Janes (jeff.janes@gmail.com) wrote:
> On Sun, Jan 22, 2017 at 6:57 AM, Stephen Frost <sfrost@snowman.net> wrote:
> > Just to make sure anyone reading the mailing list archives isn't
> > confused, running pg_start_backup does *not* make PG stop writing to
> > BASEDIR (or DATADIR, or anything, really). PG *will* continue to write
> > data into BASEDIR after pg_start_backup has been called.
>
> Correct. Unfortunately it is a very common myth that it does cause
> PostgreSQL to stop writing to the base dir.
I would love a way to dispel that myth. :/
If you have any suggestions of how we could improve the docs, I'd
certainly be happy to take a look and try to help.
> > The only thing that pg_start_backup does is identify an entry in the WAL
> > stream, from which point all WAL must be replayed when restoring the
> > backup. All WAL generated from that point (pg_start_backup point) until
> > the pg_stop_backup point *must* be replayed when restoring the backup or
> > the database will not be consistent.
>
> pg_start_backup also forces full_page_writes to be effectively 'on' for the
> duration of the backup, if it is not already explicitly on (which it
> usually will already be). This affects pg_xlog, of course, not base. But
> it is an essential step for people who run with full_page_writes=off, as it
> ensures that anything in base which got changed mid-copy will be fixed up
> during replay of the WAL.
Agreed.
Thanks!
Stephen
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To: pgsql-performance@postgresql.org
Cc: sfrost@snowman.net, jeff.janes@gmail.com, julyanto@equnix.co.id, Dinesh.Chandra@cyient.com
Subject: Re: Backup taking long time !!!
In-Reply-To: <20170123172301.GH18360@tamriel.snowman.net>
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