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.com
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