Re: Hot Standby 0.2.1

On Wed, 2009-09-23 at 17:45 +0300, Heikki Linnakangas wrote:
> Heikki Linnakangas wrote:
> > Simon Riggs wrote:
> >> On Wed, 2009-09-23 at 11:13 +0300, Heikki Linnakangas wrote:
> >>> I note that we don't emit RunningXacts after a shutdown checkpoint. So
> >>> if recovery starts at a shutdown checkpoint, we don't let read-only
> >>> backends in until the first online checkpoint. Could we treat a shutdown
> >>> checkpoint as a snapshot with no transactions running? Or do prepared
> >>> transactions screw that up?
> >> We could, but I see no requirement for starting HS from a backup taken
> >> on a shutdown database. It's just another special case to test and since
> >> we already have significant number of important test cases I'd say add
> >> this later.
> >
> > There's also a related issue that if a backend holding
> > AccessExclusiveLock crashes without writing an abort WAL record, the
> > lock is never released in the standby. We handle the expiration of xids
> > at replay of running-xacts records, but AFAICS we don't do that for locks.
>
> Ah, scratch that, I now see that we do call
> XactClearRecoveryTransactions() when we see a shutdown checkpoint, which
> clears all recovery locks. But doesn't that prematurely clear all locks
> belonging to prepared transactions as well?

Much better to read your second post(s). :-)

Yes, you have found a(nother) issue. This was the first one that gave me
pause to think of the answer. The locks currently aren't tracked as to
whether they are 2PC or not, so we would need to store that info also so
that we can selectively release locks later.

Question: is it possible to do a fast shutdown when we have a prepared
transaction? Would it be better to take a different approach there for
prepared transactions? It seems strange to write a shutdown checkpoint
when the system isn't yet "clean".

--
Simon Riggs www.2ndQuadrant.com