Ahead of the Pack: the Pacemaker High-Availability Stack
This configuration protects against both resource and node failure. If one of the virtual domains crashes, Pacemaker recovers the KVM instance in place. If a whole node goes down, Pacemaker reshuffles the resources so the remaining nodes take over the services that the failed node hosted. In the screen dump below, charlie has failed and bob has duly taken over the virtual machine that charlie had hosted:
============ Last updated: Sat Feb 4 16:18:00 2012 Stack: openais Current DC: bob - partition with quorum Version: 1.1.6-4.el6-89678d4947c5bd466e2f31acd58ea4e1edb854d5 3 Nodes configured, 2 expected votes 9 Resources configured. ============ Online: [ alice bob ] OFFLINE: [ charlie ] Full list of resources: Clone Set: cl_iscsi [p_iscsi] Started: [ alice bob ] Stopped: [ p_iscsi:2 ] p_ipmi_alice (stonith:external/ipmi): Started bob p_ipmi_bob (stonith:external/ipmi): Started alice p_ipmi_charlie (stonith:external/ipmi): Started alice p_xray (ocf::heartbeat:VirtualDomain): Started bob p_yankee (ocf::heartbeat:VirtualDomain): Started bob p_zulu (ocf::heartbeat:VirtualDomain): Started alice
Figure 1. Normal operation; virtual domains spread across all three cluster nodes.
Figure 2. Node charlie has failed; alice has automatically taken over virtual domain zulu.
Once the host charlie recovers, resources can optionally shift back to the recovered host automatically, or they can stay in place until an administrator reassigns them at the time of her choosing.
In this article, I barely scratched the surface of the Linux high-availability stack's capabilities. Pacemaker supports a diverse set of recovery policies, resource placement strategies and cluster constraints, making the stack enormously powerful.
Node: in cluster terminology, any system (typically a server) that participates in cluster communications and can potentially host cluster resources.
Fencing: a means of coordinating access to shared resources in the face of communications failure. Once a node stops responding to cluster messages unexpectedly (as opposed to gracefully signing off), other nodes shut it down to ensure it no longer has access to any shared resources. Usually enabled by making an out-of-band connection to the offending node and flipping the virtual power switch, IPMI-over-LAN being the most widely used implementation.
Resource: anything that a cluster typically manages. Resources can be very diverse, from simple IP addresses to complex database instances or dæmons.
Ring: in Totem protocol terminology, one of the (typically redundant) links over which cluster messaging communicates.
"The Totem Single-Ring Ordering and Membership Protocol" (research paper explaining the Totem protocol): http://www.cs.jhu.edu/~yairamir/tocs.ps
"Clusters From Scratch" (hands-on documentation for Pacemaker novices): http://www.clusterlabs.org/doc/en-US/Pacemaker/1.1/html/Clusters_from_Scratch
"Pacemaker Configuration Explained" (reference documentation of Pacemaker configuration internals, not for the faint at heart): http://www.clusterlabs.org/doc/en-US/Pacemaker/1.1/html/Pacemaker_Explained
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