Re: "Hot standby"?

On Tue, Aug 11, 2009 at 12:30:58PM +0300, Peter Eisentraut wrote:

> warm - If the first node dies, the replacement node needs to do some work to
> get ready, but it's a lot quicker than "cold".
>
> hot - If the first node dies, the replacement node can take over immediately.

> For example, I'd say that a DRBD-based solution would be a cold standby.
> Among WAL-based solutions, what we have now with pg_standby (nomen est omen),
> is a warmish standby. From what I understand, Simon's patch set does not
> change the "warm" property of this arrangement at all. It only changes the
> "standby" to a "slave".

> Am I off? What other definition of terms justifies the description of "hot
> standby"?

As for "warm/hot", it depends on what you exactly mean with "get
ready":

(A) If you mean "it is possible to connect to the second node", then
Simon's patch is "hot".

IIRC this meaning agrees with the terminology used by other
databases, such as Oracle, Sybase, etc.

(B) If you mean "the contents of the second node are equal to the
contents of the first node", then Simon's patch is "warm".

But then we have by definition "hot" == "synchronous" and "either
warm or cold" == "asynchronous". IMHO the other case is
preferrable, as the word sets "synchronous/asynchronous" and
"cold/warm/hot" are more expressive because they measure two
independent properties.

Best regards,
Dr. Gianni Ciolli - 2ndQuadrant Italia
PostgreSQL Training, Services and Support
gianni(dot)ciolli(at)2ndquadrant(dot)it | www.2ndquadrant.it