Re: Offline enabling/disabling of data checksums

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

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

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