E.3. Changing Consensus Timeout
The consensus timeout specifies the time (in milliseconds) to wait for consensus to be achieved before starting a new round of membership configuration.
When consensus is calculated automatically, the following rules will be used:
- If configuring a cluster of 2 or less nodes, consensus will be
(token * 0.2), with a maximum of 2000 milliseconds and a minimum of 200 milliseconds. - If configuring a cluster of 3 or more nodes, consensus will be
(token + 2000 milliseconds)
If you let
cman configure your consensus timeout in this fashion, realize that moving from 2 to 3 (or more) nodes will require a cluster restart, since the consensus timeout will need to change to the larger value based on the token timeout.
When configuring a 2-member cluster with the ultimate intention of adding more nodes at a later time, you must adjust the consensus timeout so that you do not have to restart the cluster to add the new nodes. To do this, you can edit the
cluster.conf as follows:
<totem token="X" consensus="X + 2000" />
Note that the configuration parser does not calculate
X + 2000 automatically. An integer value must be used rather than an equation.
The advantage of the optimized consensus timeout for 2 node clusters is that overall failover time is reduced for the 2 node case since consensus is not a function of the token timeout.
Note
For two node auto-detection in
cman, the number of physical nodes matters and not the presence of the two_node=1 directive in cluster.conf.
