Re: Transaction Snapshots and Hot Standby

On Thu, 2008-09-11 at 15:42 +0300, Heikki Linnakangas wrote:
> Gregory Stark wrote:
> > b) vacuum on the server which cleans up a tuple the slave has in scope has to
> >    block WAL reply on the slave (which I suppose defeats the purpose of having
> >    a live standby for users concerned more with fail-over latency).
> 
> One problem with this, BTW, is that if there's a continuous stream of 
> medium-length transaction in the slave, each new snapshot taken will 
> prevent progress in the WAL replay, so the WAL replay will advance in 
> "baby steps", and can fall behind indefinitely. As soon as there's a 
> moment that there's no active snapshot, it can catch up, but if the 
> slave is seriously busy, that might never happen.

It should be possible to do mixed mode.

Stall WAL apply for up to X seconds, then cancel queries. Some people
may want X=0 or low, others might find X = very high acceptable (Merlin
et al).

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support