From 806ef12d98aaf596661007cd36b0a3105469396a Mon Sep 17 00:00:00 2001 From: Dietmar Maurer Date: Mon, 22 Aug 2016 12:51:17 +0200 Subject: [PATCH] add section about quorum and cluster cold start --- pvecm.adoc | 44 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 44 insertions(+) diff --git a/pvecm.adoc b/pvecm.adoc index 47e382f..6f8b382 100644 --- a/pvecm.adoc +++ b/pvecm.adoc @@ -281,6 +281,50 @@ 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' waits up to 60 seconds to reach +quorum, and then starts all guests. If it fails to get quorum, that +service simply aborts, and you need to start your guest manually once +you have quorum. + +If you start all nodes at the same time (for example when power comes +back), it is likely that you reach quorum within above timeout. But +startup can fail if some nodes starts much faster than others, so you +need to start your guest manually after reaching quorum. You can do +that on the GUI, or on the command line with: + + systemctl start pve-manager + + ifdef::manvolnum[] include::pve-copyright.adoc[] endif::manvolnum[] -- 2.39.2