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author | Jeff Darcy <jdarcy@redhat.com> | 2014-04-22 15:37:09 +0000 |
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committer | Jeff Darcy <jdarcy@redhat.com> | 2014-04-22 15:37:09 +0000 |
commit | a827c5eab32a43ade5551259ea56a6a1af7e861b (patch) | |
tree | e6707df68f72baa8645210ba931272285116ad85 /doc/features/server-quorum.md | |
parent | 46d333783a968ab39e0beade9c7a1eec8035f8b1 (diff) | |
parent | 99bfc2a2a1689da1e173cb2f8ef54d2b09ef3a5d (diff) |
Merge branch 'upstream'
Conflicts:
glusterfs.spec.in
xlators/mgmt/glusterd/src/Makefile.am
xlators/mgmt/glusterd/src/glusterd-utils.c
xlators/mgmt/glusterd/src/glusterd.h
Change-Id: I27bdcf42b003cfc42d6ad981bd2bf8180176806d
Diffstat (limited to 'doc/features/server-quorum.md')
-rw-r--r-- | doc/features/server-quorum.md | 44 |
1 files changed, 44 insertions, 0 deletions
diff --git a/doc/features/server-quorum.md b/doc/features/server-quorum.md new file mode 100644 index 000000000..7b20084ce --- /dev/null +++ b/doc/features/server-quorum.md @@ -0,0 +1,44 @@ +# Server Quorum + +Server quorum is a feature intended to reduce the occurrence of "split brain" +after a brick failure or network partition. Split brain happens when different +sets of servers are allowed to process different sets of writes, leaving data +in a state that can not be reconciled automatically. The key to avoiding split +brain is to ensure that there can be only one set of servers - a quorum - that +can continue handling writes. Server quorum does this by the brutal but +effective means of forcing down all brick daemons on cluster nodes that can no +longer reach enough of their peers to form a majority. Because there can only +be one majority, there can be only one set of bricks remaining, and thus split +brain can not occur. + +## Options + +Server quorum is controlled by two parameters: + + * **cluster.server-quorum-type** + + This value may be "server" to indicate that server quorum is enabled, or + "none" to mean it's disabled. + + * **cluster.server-quorum-ratio** + + This is the percentage of cluster nodes that must be up to maintain quorum. + More precisely, this percentage of nodes *plus one* must be up. + +Note that these are cluster-wide flags. All volumes served by the cluster will +be affected. Once these values are set, quorum actions - starting or stopping +brick daemons in response to node or network events - will be automatic. + +## Best Practices + +If a cluster with an even number of nodes is split exactly down the middle, +neither half can have quorum (which requires **more than** half of the total). +This is particularly important when N=2, in which case the loss of either node +leads to loss of quorum. Therefore, it is highly advisable to ensure that the +cluster size is three or greater. The "extra" node in this case need not have +any bricks or serve any data. It need only be present to preserve the notion +of a quorum majority less than the entire cluster membership, allowing the +cluster to survive the loss of a single node without losing quorum. + + + |