pct: merge wiki content about bind mounts

[pve-docs.git] / pvecm.adoc

diff --git a/pvecm.adoc b/pvecm.adoc
index 47e382fb1638d2fe98c48ac466da8e7734e3bb9b..e6e2058d73ed813d272d2a91dbe692eb6b51a2ec 100644 (file)
--- a/pvecm.adoc
+++ b/pvecm.adoc
@@ -281,6 +281,45 @@ cluster again, you have to
 * then join it, as explained in the previous section.
 
 
+Quorum
+------
+
+{pve} use a quorum-based technique to provide a consistent state among
+all cluster nodes.
+
+[quote, from Wikipedia, Quorum (distributed computing)]
+____
+A quorum is the minimum number of votes that a distributed transaction
+has to obtain in order to be allowed to perform an operation in a
+distributed system.
+____
+
+In case of network partitioning, state changes requires that a
+majority of nodes are online. The cluster switches to read-only mode
+if it loose quorum.
+
+NOTE: {pve} assigns a single vote to each node by default.
+
+
+Cluster Cold Start
+------------------
+
+It is obvious that a cluster is not quorate when all nodes are
+offline. This is a common case after a power failure.
+
+NOTE: It is always a good idea to use an uninterruptible power supply
+('UPS', also called 'battery backup') to avoid this state. Especially if
+you want HA.
+
+On node startup, service 'pve-manager' is started and waits for
+quorum. Once quorate, it starts all guests which have the 'onboot'
+flag set.
+
+When you turn on nodes, or when power comes back after power failure,
+it is likely that some nodes boots faster than others. Please keep in
+mind that guest startup is delayed until you reach quorum.
+
+
 ifdef::manvolnum[]
 include::pve-copyright.adoc[]
 endif::manvolnum[]