Provided by: corosync_2.4.3-0ubuntu1.3_amd64 
NAME
votequorum - Votequorum Configuration Overview
OVERVIEW
The votequorum service is part of the corosync project. This service can be optionally
loaded into the nodes of a corosync cluster to avoid split-brain situations. It does this
by having a number of votes assigned to each system in the cluster and ensuring that only
when a majority of the votes are present, cluster operations are allowed to proceed. The
service must be loaded into all nodes or none. If it is loaded into a subset of cluster
nodes the results will be unpredictable.
The following corosync.conf extract will enable votequorum service within corosync:
quorum {
provider: corosync_votequorum
}
votequorum reads its configuration from corosync.conf. Some values can be changed at
runtime, others are only read at corosync startup. It is very important that those values
are consistent across all the nodes participating in the cluster or votequorum behavior
will be unpredictable.
votequorum requires an expected_votes value to function, this can be provided in two ways.
The number of expected votes will be automatically calculated when the nodelist { }
section is present in corosync.conf or expected_votes can be specified in the quorum { }
section. Lack of both will disable votequorum. If both are present at the same time, the
quorum.expected_votes value will override the one calculated from the nodelist.
Example (no nodelist) of an 8 node cluster (each node has 1 vote):
quorum {
provider: corosync_votequorum
expected_votes: 8
}
Example (with nodelist) of a 3 node cluster (each node has 1 vote):
quorum {
provider: corosync_votequorum
}
nodelist {
node {
ring0_addr: 192.168.1.1
}
node {
ring0_addr: 192.168.1.2
}
node {
ring0_addr: 192.168.1.3
}
}
SPECIAL FEATURES
two_node: 1
Enables two node cluster operations (default: 0).
The "two node cluster" is a use case that requires special consideration. With a standard
two node cluster, each node with a single vote, there are 2 votes in the cluster. Using
the simple majority calculation (50% of the votes + 1) to calculate quorum, the quorum
would be 2. This means that the both nodes would always have to be alive for the cluster
to be quorate and operate.
Enabling two_node: 1, quorum is set artificially to 1.
Example configuration 1:
quorum {
provider: corosync_votequorum
expected_votes: 2
two_node: 1
}
Example configuration 2:
quorum {
provider: corosync_votequorum
two_node: 1
}
nodelist {
node {
ring0_addr: 192.168.1.1
}
node {
ring0_addr: 192.168.1.2
}
}
NOTES: enabling two_node: 1 automatically enables wait_for_all. It is still possible to
override wait_for_all by explicitly setting it to 0. If more than 2 nodes join the
cluster, the two_node option is automatically disabled.
wait_for_all: 1
Enables Wait For All (WFA) feature (default: 0).
The general behaviour of votequorum is to switch a cluster from inquorate to quorate as
soon as possible. For example, in an 8 node cluster, where every node has 1 vote,
expected_votes is set to 8 and quorum is (50% + 1) 5. As soon as 5 (or more) nodes are
visible to each other, the partition of 5 (or more) becomes quorate and can start
operating.
When WFA is enabled, the cluster will be quorate for the first time only after all nodes
have been visible at least once at the same time.
This feature has the advantage of avoiding some startup race conditions, with the cost
that all nodes need to be up at the same time at least once before the cluster can
operate.
A common startup race condition based on the above example is that as soon as 5 nodes
become quorate, with the other 3 still offline, the remaining 3 nodes will be fenced.
It is very useful when combined with last_man_standing (see below).
Example configuration:
quorum {
provider: corosync_votequorum
expected_votes: 8
wait_for_all: 1
}
last_man_standing: 1 / last_man_standing_window: 10000
Enables Last Man Standing (LMS) feature (default: 0). Tunable last_man_standing_window
(default: 10 seconds, expressed in ms).
The general behaviour of votequorum is to set expected_votes and quorum at startup (unless
modified by the user at runtime, see below) and use those values during the whole lifetime
of the cluster.
Using for example an 8 node cluster where each node has 1 vote, expected_votes is set to 8
and quorum to 5. This condition allows a total failure of 3 nodes. If a 4th node fails,
the cluster becomes inquorate and it will stop providing services.
Enabling LMS allows the cluster to dynamically recalculate expected_votes and quorum under
specific circumstances. It is essential to enable WFA when using LMS in High Availability
clusters.
Using the above 8 node cluster example, with LMS enabled the cluster can retain quorum and
continue operating by losing, in a cascade fashion, up to 6 nodes with only 2 remaining
active.
Example chain of events:
1) cluster is fully operational with 8 nodes.
(expected_votes: 8 quorum: 5)
2) 3 nodes die, cluster is quorate with 5 nodes.
3) after last_man_standing_window timer expires,
expected_votes and quorum are recalculated.
(expected_votes: 5 quorum: 3)
4) at this point, 2 more nodes can die and
cluster will still be quorate with 3.
5) once again, after last_man_standing_window
timer expires expected_votes and quorum are
recalculated.
(expected_votes: 3 quorum: 2)
6) at this point, 1 more node can die and
cluster will still be quorate with 2.
7) one more last_man_standing_window timer
(expected_votes: 2 quorum: 2)
NOTES: In order for the cluster to downgrade automatically from 2 nodes to a 1 node
cluster, the auto_tie_breaker feature must also be enabled (see below). If
auto_tie_breaker is not enabled, and one more failure occurs, the remaining node will not
be quorate. LMS does not work with asymmetric voting schemes, each node must vote 1. LMS
is also incompatible with quorum devices, if last_man_standing is specified in
corosync.conf then the quorum device will be disabled.
Example configuration 1:
quorum {
provider: corosync_votequorum
expected_votes: 8
last_man_standing: 1
}
Example configuration 2 (increase timeout to 20 seconds):
quorum {
provider: corosync_votequorum
expected_votes: 8
last_man_standing: 1
last_man_standing_window: 20000
}
auto_tie_breaker: 1
Enables Auto Tie Breaker (ATB) feature (default: 0).
The general behaviour of votequorum allows a simultaneous node failure up to 50% - 1 node,
assuming each node has 1 vote.
When ATB is enabled, the cluster can suffer up to 50% of the nodes failing at the same
time, in a deterministic fashion. By default the cluster partition, or the set of nodes
that are still in contact with the node that has the lowest nodeid will remain quorate.
The other nodes will be inquorate. This behaviour can be changed by also specifying
auto_tie_breaker_node: lowest|highest|<list of node IDs>
´lowest' is the default, 'highest' is similar in that if the current set of nodes contains
the highest nodeid then it will remain quorate. Alternatively it is possible to specify a
particular node ID or list of node IDs that will be required to maintain quorum. If a
(space-separated) list is given, the nodes are evaluated in order, so if the first node is
present then it will be used to determine the quorate partition, if that node is not in
either half (ie was not in the cluster before the split) then the second node ID will be
checked for and so on. ATB is incompatible with quorum devices - if auto_tie_breaker is
specified in corosync.conf then the quorum device will be disabled.
Example configuration 1:
quorum {
provider: corosync_votequorum
expected_votes: 8
auto_tie_breaker: 1
auto_tie_breaker_node: lowest
}
Example configuration 2:
quorum {
provider: corosync_votequorum
expected_votes: 8
auto_tie_breaker: 1
auto_tie_breaker_node: 1 3 5
}
allow_downscale: 1
Enables allow downscale (AD) feature (default: 0).
THIS FEATURE IS INCOMPLETE AND CURRENTLY UNSUPPORTED.
The general behaviour of votequorum is to never decrease expected votes or quorum.
When AD is enabled, both expected votes and quorum are recalculated when a node leaves the
cluster in a clean state (normal corosync shutdown process) down to configured
expected_votes.
Example use case:
1) N node cluster (where N is any value higher than 3)
2) expected_votes set to 3 in corosync.conf
3) only 3 nodes are running
4) admin requires to increase processing power and adds 10 nodes
5) internal expected_votes is automatically set to 13
6) minimum expected_votes is 3 (from configuration)
- up to this point this is standard votequorum behavior -
7) once the work is done, admin wants to remove nodes from the cluster
8) using an ordered shutdown the admin can reduce the cluster size
automatically back to 3, but not below 3, where normal quorum
operation will work as usual.
Example configuration:
quorum {
provider: corosync_votequorum
expected_votes: 3
allow_downscale: 1
}
allow_downscale implicitly enabled EVT (see below).
expected_votes_tracking: 1
Enables Expected Votes Tracking (EVT) feature (default: 0).
Expected Votes Tracking stores the highest-seen value of expected votes on disk and uses
that as the minimum value for expected votes in the absence of any higher authority (eg a
current quorate cluster). This is useful for when a group of nodes becomes detached from
the main cluster and after a restart could have enough votes to provide quorum, which can
happen after using allow_downscale.
Note that even if the in-memory version of expected_votes is reduced, eg by removing nodes
or using corosync-quorumtool, the stored value will still be the highest value seen - it
never gets reduced.
The value is held in the file /var/lib/corosync/ev_tracking which can be deleted if you
really do need to reduce the expected votes for any reason, like the node has been moved
to a different cluster.
VARIOUS NOTES
* WFA / LMS / ATB / AD can be used combined together.
* In order to change the default votes for a node there are two options:
1) nodelist:
nodelist {
node {
ring0_addr: 192.168.1.1
quorum_votes: 3
}
....
}
2) quorum section (deprecated):
quorum {
provider: corosync_votequorum
expected_votes: 2
votes: 2
}
In the event that both nodelist and quorum { votes: } are defined, the value from the
nodelist will be used.
* Only votes, quorum_votes, expected_votes and two_node can be changed at runtime.
Everything else requires a cluster restart.
BUGS
No known bugs at the time of writing. The authors are from outerspace. Deal with it.
SEE ALSO
corosync(8), corosync.conf(5), corosync-quorumtool(8), corosync-qdevice(8),
votequorum_overview(8)