Fix carp FAQ: net.inet.carp.preempt is not necessary for group demotion failover

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Fix carp FAQ: net.inet.carp.preempt is not necessary for group demotion failover

Christopher Zimmermann-2
Hi,

if I read the code correctly, the demotion counter would only _prevent_
preemptive failover if the preempting master was demoted.
A demoted master would failover to a less demoted backup no matter what
the advbase / advskew timing says.

The relevant code is following sys/netinet/ip_carp.c:665.

Here's the diff:

--- carp.html.orig Sun Nov 12 11:26:42 2017
+++ carp.html Sun Nov 12 11:33:05 2017
@@ -194,8 +194,12 @@
     By default, all carp(4) interfaces are added to the <tt>carp</tt> group.
     Each group has a <tt>carpdemote</tt> counter affecting all carp(4)
     interfaces belonging to that group.
-    As described below, it can be useful to group certain interfaces together
-    for failover purposes.
+    This enables failing over the whole group of interfaces in the event that
+    one interface goes down.
+    If one physical CARP-enabled interface goes down, CARP will increase
+    the demotion counter, <tt>carpdemote</tt>, by 1 on interface groups that
+    the carp(4) interface is a member of, in effect causing all group
+    members to fail-over together.
 
 <dt><tt><i>ipaddress</i></tt>
 <dd>This is the shared IP address assigned to the redundancy group.
@@ -219,12 +223,6 @@
 <dt><tt>net.inet.carp.preempt</tt>
 <dd>Allow hosts within a redundancy group that have a better
     <tt>advbase</tt> and <tt>advskew</tt> to preempt the master.
-    In addition, this option also enables failing over a group of interfaces
-    together in the event that one interface goes down.
-    If one physical CARP-enabled interface goes down, CARP will increase
-    the demotion counter, <tt>carpdemote</tt>, by 1 on interface groups that
-    the carp(4) interface is a member of, in effect causing all group
-    members to fail-over together.
     <tt>net.inet.carp.preempt</tt> is 0 (disabled) by default.
 
 <dt><tt>net.inet.carp.log</tt>
@@ -414,7 +412,7 @@
 Configure fw1:
 
 <blockquote><pre>
-! enable preemption and group interface failover
+! enable preemption
 # <b>sysctl net.inet.carp.preempt=1</b>
 # <b>echo 'net.inet.carp.preempt=1' >> /etc/sysctl.conf</b>
 
@@ -437,7 +435,7 @@
 Configure fw2:
 
 <blockquote><pre>
-! enable preemption and group interface failover
+! enable preemption
 # <b>sysctl net.inet.carp.preempt=1</b>
 # <b>echo 'net.inet.carp.preempt=1' >> /etc/sysctl.conf</b>



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