Re: tar error while running basebackup

> From: Andre Brandt <brandt(at)decoit(dot)de>
> Subject: [GENERAL] tar error while running basebackup
> To: pgsql-general(at)postgresql(dot)org
> Date: Tuesday, 13 October, 2009, 11:40 AM
> Hi!
>
> We're using two backup strategies to get consistent backups
> of our postgresql databases. First, we create a complete
> dump every night by running pg_dump, zipping the file and
> writing this backup on tape.
> Second, we create a basebackup every saturday. To create a
> basebackup, we run pg_start_backup. After that, we create a
> tar file of the complete database directory and stop the
> backup mode by running pg_stop_backup. Of course, all
> archived wal logs are also copied ;)
>
> Well, everything was fine for month. But from time to time,
> I get an error when running tar:
>
> tar: ./base/208106/209062: File shrank by 262144 bytes;
> padding with zeros
> tar: ./base/208106/210576: file changed as we read it
> tar: ./base/208106/210577: file changed as we read it
> tar: ./base/208106/210431: file changed as we read it
>
> How can this happen? I always thought, that, when in backup
> mode, nothing is able to change the database - so the
> database files shouldn't change. Can autovaccumdb cause the
> changes?
> I already read something, that this kind of errors can be
> ignored when creating a basebackup, but I can't believe
> that. Normally, the tar file have to be worthless, when an
> error occurs - or do I have an error in reasoning?

My understanding was that when you back up like this you are actually taring up inconsistant database files, but it doesn't matter as you have the wal logs to replay any changes, thus correcting the inconsistancies.