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"Hot standby"?

From: Peter Eisentraut <peter_e(at)gmx(dot)net>
To: pgsql-hackers(at)postgresql(dot)org
Subject: "Hot standby"?
Date: 2009-08-11 09:30:58
Message-ID: 200908111230.58135.peter_e@gmx.net (view raw or flat)
Thread:
Lists: pgsql-hackers
What is "hot" and "standby" about the proposed "hot standby" feature?

The way I understand these terms in a replication/cluster scenario are:

cold - If the first node dies, you need to start the replacement node from a 
standing start.

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.

standby - While the master node is running, the standby node instance cannot 
be used for anything (useful).

slave - While the master node is running, the slave node can be used in 
limited capacity (typically read-only).

master - Both/all nodes have equivalent capabilities all the time while the 
cluster is up.


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"?

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