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.

Glossary

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.

Resources

"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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Its said changes charge to be

Tyrwhittse's picture

Its said changes charge to be fabricated because of its brokenness. Boloney! Get aback in the MB star C3 drivers bench and acquisition a distro that puts you aback abaft the caster and not these distributions that accept put aqueduct

Interesting

tires's picture

This one is quite an interesting read. Thanks for sharing.

HA Documentation

Adrie T's picture

Where can we find a more detail documentation that easy to undertand about Linux HA beside the link that you have mention above, especially about Corosync and Pacemaker.
The two documentation "Cluster from Scratch" dan "Pacemaker Explained" was too short and directly jump start to the configuration without any explanation about how those two technology works and step by step configuration.
If there is any documentation like http://www.drbd.org/home/what-is-drbd/ that explained DRBD in an easy way it will be a great help to understand about PaceMaker and Corosync.
It seems that very few good documentation about Linux HA available everywhere.

Reply to comment | Linux Journal

[need money|how to make money on the side|money making|'s picture

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and found that it's really informative. I am gonna watch out for brussels. I'll appreciate if you continue this in future. Many people will be benefited from your writing. Cheers!

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