Re: [HACKERS] WAL logging freezing

Simon Riggs wrote:
> On Mon, 2006-10-30 at 12:05 -0500, Tom Lane wrote:
> > Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> > > Ugh. Is there another solution to this? Say, sync the buffer so that
> > > the hint bits are written to disk?
> >
> > Yeah. The original design for all this is explained by the notes for
> > TruncateCLOG:
> >
> > * When this is called, we know that the database logically contains no
> > * reference to transaction IDs older than oldestXact. However, we must
> > * not truncate the CLOG until we have performed a checkpoint, to ensure
> > * that no such references remain on disk either; else a crash just after
> > * the truncation might leave us with a problem.
> >
> > The pre-8.2 coding is actually perfectly safe within a single database,
> > because TruncateCLOG is only called at the end of a database-wide
> > vacuum, and so the checkpoint is guaranteed to have flushed valid hint
> > bits for all tuples to disk. There is a risk in other databases though.
> > I think that in the 8.2 structure the equivalent notion must be that
> > VACUUM has to flush and fsync a table before it can advance the table's
> > relminxid.
>
> Ouch! We did discuss that also. Flushing the buffercache is nasty with
> very large caches, so this makes autovacuum much less friendly - and
> could take a seriously long time if you enforce the vacuum delay
> costings.
>
> ISTM we only need to flush iff the clog would be truncated when we
> update relminxid. Otherwise we are safe to update even if we crash,
> since the clog will not have been truncated.

I don't understand. When clog is actually going to be truncated, if
it's determined that there's any page that can be truncated, then a
checkpoint is forced. If no page is going to be removed then there's no
checkpoint, which makes a lot of sense and of course avoids the problem
of useless flushes.

In fact I don't understand what's the point about multiple databases vs.
a single database. Surely a checkpoint would flush all buffers in all
databases, no? This would flush all hint bits, everywhere. So this bug
does not really exist.

--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support