Message-ID: <1546958962.32387.13.camel@credativ.de>
Subject: Re: Offline enabling/disabling of data checksums
From: Michael Banck <michael.banck@credativ.de>
To: Bernd Helmle <mailings@oopsware.de>, Fabien COELHO <coelho@cri.ensmp.fr>
Cc: Magnus Hagander <magnus@hagander.net>, Michael Paquier
 <michael@paquier.xyz>,  Postgres hackers <pgsql-hackers@postgresql.org>
Date: Tue, 08 Jan 2019 15:49:22 +0100
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	 <1546945543.32387.8.camel@credativ.de>
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Am Dienstag, den 08.01.2019, 15:39 +0100 schrieb Bernd Helmle:
> Am Dienstag, den 08.01.2019, 15:09 +0100 schrieb Fabien COELHO:
> > > The question is how to reliably do this in an acceptable way? Just
> > > faking a postmaster.pid sounds pretty hackish to me, do you have
> > > any
> > > suggestions here?
> > 
> > Adding a new state to ControlFileData which would prevent it from 
> > starting?
> 
> But then you have to make sure the control flag gets cleared in any
> case pg_verify_checksums crashes somehow or gets SIGKILL'ed ...
> 
> Setting the checksum flag is done after having finished all blocks, so
> there is no problem. But we need to set this new flag before and reset
> it afterwards, so in between strange things can happen (as the various
> calls to exit() within error handling illustrates). 

It seems writing a note like "pg_checksums is running" into the
postmaster.pid would work, and would give a hopefully useful hint to
somebody trying to start Postgres while pg_checksums is running:

postgres@kohn:~$ echo  "pg_checksums running with pid 1231, cluster disabled" > data/postmaster.pid 
postgres@kohn:~$ pg_ctl -D data -l logfile start
pg_ctl: invalid data in PID file "data/postmaster.pid"
postgres@kohn:~$ echo $?
1
postgres@kohn:~$ 

If the DBA then just simply deletes postmaster.pid and starts over, well
then I call pilot error; though we could in theory change pg_ctl (or
whatever checks postmaster.pid) to emit an even more useful error
message if it encounters a "cluster is locked" keyword in it.

Not sure whether everybody likes that (or is future-proof for that
matter), but I like it better than adding a new field to the control
file, for the reasons Bernd outlined above.


Michael

-- 
Michael Banck
Projektleiter / Senior Berater
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