Re: Offline enabling/disabling of data checksums

Hi,

Am Freitag, den 15.03.2019, 11:50 +0900 schrieb Michael Paquier:
> On Thu, Mar 14, 2019 at 04:26:20PM +0100, Michael Banck wrote:
> > Am Donnerstag, den 14.03.2019, 15:26 +0100 schrieb Magnus Hagander:
> > > One big-hammer method could be similar to what pg_upgrade does --
> > > temporarily rename away the controlfile so postgresql can't start, and
> > > when done, put it back.
> > 
> > That sounds like a good solution to me. I've made PoC patch for that,
> > see attached.
> 
> Indeed.  I did not know this trick from pg_upgrade.  We could just use
> the same.
> 
> > The only question is whether pg_checksums should try to move pg_control
> > back (i) on failure (ii) when interrupted?
> 
> Yes, we should have a callback on SIGINT and SIGTERM here which just
> moves back in place the control file if the temporary one exists.  I
> have been able to grab some time to incorporate the feedback gathered
> on this thread, and please find attached a new version of the patch to
> add --enable/--disable.  

Thanks!

One thing stood out to me while quickly looking over it:

+		/*
+		 * Flush the control file and its parent path to make the change
+		 * durable.
+		 */
+		if (fsync_fname(controlfile_path, false, progname) != 0 ||
+			fsync_parent_path(controlfile_path, progname) != 0)
+		{
+			/* errors are already logged on failure */
+			exit(1);
+		}

ISTM this would not run fsync_parent_path() unless the first fsync fails
which is not the intended use. I guess we need two ifs here?


Michael

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