Simon Riggs wrote: > On Mon, 2006-10-30 at 12:05 -0500, Tom Lane wrote: > > Alvaro Herrera <[EMAIL PROTECTED]> 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 ---------------------------(end of broadcast)--------------------------- TIP 2: Don't 'kill -9' the postmaster
