Re: Something else about Redo Logs disappearing - Mailing list pgsql-general

On Sat, Jun 13, 2020 at 01:53:28PM +0200, Laurenz Albe wrote:

! > I've never seen anybody coding bash - it is strongly shunned in the
! > Berkeley community. 
! 
! Strange, but then I don't move in these circles.

Never mind.

! > Some Questions:
! >  1. There are explicit error messages in loc-82 and -92 of pgpre.sh.
! >     To where are these written?
! 
! Standard error.  It is up to the caller of the script to route that
! somewhere useful.

Understood.

! >  2. The result data from pg_stop_backup() are stored into the living
! >     database. But, according to the docs, they should be placed into
! >     the completed backup. Do I have a misunderstanding here?
! 
! Right, but these scripts don't know anything about that backup itself.
! They are designed to be called before and after the backup.
! In between, you back up the data directory however you think fit.
! 
! It is the responsibility of the caller of the post-backup script
! to add the "backup_label" file to the backup.

I see.

! >  4. If, by misconfiguration and/or operator error, the backup system
! >     happens to start a second backup. in parallel to the first,
! >     then do I correctly assume, both backups will be rendered
! >     inconsistent while this may not be visible to the operator; and
! >     the earlier backup would be flagged as apparently successful while
! >     carrying the wrong (later) label?
! 
! If you are using my scripts and start a second backup while the first
! one is still running, the first backup will be interrupted.

This is not what I am asking. It appears correct to me, that, on
the database, the first backup will be interrupted. But on the
tape side, this might go unnoticed, and on completion it will
successfully receive the termination code from the *SECOND*
backup - which means that on tape we will have a seemingly
successful backup, which
 1. is corrupted, and
 2. carries a wrong label.

! This is specific to my scripts, PostgreSQL's non-exclusive backup
! can perform more than one concurrent backup successfully.
! I tried to keep things simple.

I understand. But the operator may not know that and/or accidentially
start a second backup while one is still running. And this will then
result in ...

! If you have the wrong "backup_label", you end up with silent data corruption.

 ... this.

Indeed this is difficult to avoid, because the high risk of silent
data corruption is an elementary architectural feature of the so-called
"new API". Which is why this is not going to run on my ship.

But you will have to wait - the to-be-expected inrush of service-sales
due to corrupted clusters will only happen after R.13 is active and
peope are *forced* to cope with that "new API".

Thanks for the effort of answering my questions.

cheerio,
PMc