Re: RFC: Add 'taint' field to pg_control.

On Thu, Mar 01, 2018 at 09:12:18AM +0800, Craig Ringer wrote:
> On 1 March 2018 at 06:28, Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:
> > The more fine grained these are the more useful they can be:
> >
> > Running with fsync=off is common advice while loading, so reporting that
> > "fsync=off at some point" is much less useful than reporting "having to run
> > recovery from an fsync=off database".
> >
> > I propose there could be two bits:
> >
> > 1. fsync was off at some point in history when the DB needed recovery;
> > 2. fsync was off during the current instance; I assume the bit would be set
> > whenever fsync=off was set, but could be cleared when the server was
> > cleanly shut down. If the bit is set during recovery, the first bit would
> > be permanently set. Not sure but if this bit is set and then fsync
> > re-enabled, maybe this could be cleared after any checkpoint and not just
> > shutdown ?
> >
> I think that's a very useful way to eliminate false positives and make this
> diagnostic field more useful. But we'd need to guarantee that when you've
> switched from fsync=off to fsync=on, we've actually fsync'd every extent of
> every table and index, whether or not we think they're currently dirty and
> whether or not the smgr currently has them open. Do something like initdb
> does to flush everything. Without that, we could have WAL-vs-heap ordering
> issues and so on *even after a Pg restart* if the system has lots of dirty
> writeback to do.

I think a similar, "2 bit" scheme *could* be applied to full_page_writes, too.
And the 2nd "soft" bit for FPW could also be cleared after checkpoint. I'd
think that for FPW, previous checkpoints which had fsync=off wouldn't need
special handling - so long as FPW=on, the "soft" bit can be cleared after
successful checkpoint (as a micro-optimization, don't clear the "soft" bit if
the "hard" bit is set).

BTW this scheme has the amusing consequence that setting fsync=off should
involve first writing+fsyncing these bits to pgcontrol.

Justin