pct: merge wiki content about bind mounts

[pve-docs.git] / pvecm.adoc

diff --git a/pvecm.adoc b/pvecm.adoc
index 84eb411bde9ec50b2283d0c07bb2f9b7df08da1f..e6e2058d73ed813d272d2a91dbe692eb6b51a2ec 100644 (file)
--- a/pvecm.adoc
+++ b/pvecm.adoc
@@ -59,7 +59,7 @@ Requirements
 * All nodes must be in the same network as corosync uses IP Multicast
  to communicate between nodes (also see
  http://www.corosync.org[Corosync Cluster Engine]). Corosync uses UDP
- ports 5404and 5405 for cluster communication.
+ ports 5404 and 5405 for cluster communication.
 +
 NOTE: Some switches do not support IP multicast by default and must be
 manually enabled first.
@@ -89,7 +89,6 @@ hostname and IP is not possible after cluster creation.
 Currently the cluster creation has to be done on the console, so you
 need to login via 'ssh'.
 
-
 Create the Cluster
 ------------------
 
@@ -98,6 +97,10 @@ your cluster. This name cannot be changed later.
 
  hp1# pvecm create YOUR-CLUSTER-NAME
 
+CAUTION: The cluster name is used to compute the default multicast
+address. Please use unique cluster names if you run more than one
+cluster inside your network.
+
 To check the state of your cluster use:
 
  hp1# pvecm status
@@ -113,10 +116,10 @@ Login via 'ssh' to the node you want to add.
 For `IP-ADDRESS-CLUSTER` use the IP from an existing cluster node.
 
 CAUTION: A new node cannot hold any VM´s, because you would get
-conflicts about identical VM IDs. Also, all existing configuration is
-overwritten when you join a new node to the cluster. To workaround,
-use vzdump to backup and restore to a different VMID after adding
-the node to the cluster.
+conflicts about identical VM IDs. Also, all existing configuration in
+'/etc/pve' is overwritten when you join a new node to the cluster. To
+workaround, use vzdump to backup and restore to a different VMID after
+adding the node to the cluster.
 
 To check the state of cluster:
 
@@ -179,7 +182,7 @@ Move all virtual machines from the node. Make sure you have no local
 data or backups you want to keep, or save them accordingly.
 
 Log in to one remaining node via ssh. Issue a 'pvecm nodes' command to
-identify the nodeID:
+identify the node ID:
 
 ----
 hp1# pvecm status
@@ -278,6 +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[]