kexec-tools/kdump-in-cluster-environment.txt

Kdump-in-cluster-environment HOWTO

Introduction

Kdump is a kexec based crash dumping mechansim for Linux. This docuement 
illustrate how to configure kdump in cluster environment to allow the kdump 
crash recovery service complete without being preempted by traditional power
fencing methods. 

Overview

Kexec/Kdump

Details about Kexec/Kdump are available in Kexec-Kdump-howto file and will not
be described here.

fence_kdump

fence_kdump is an I/O fencing agent to be used with the kdump crash recovery 
service. When the fence_kdump agent is invoked, it will listen for a message 
from the failed node that acknowledges that the failed node is executing the 
kdump crash kernel. Note that fence_kdump is not a replacement for traditional
fencing methods. The fence_kdump agent can only detect that a node has entered
the kdump crash recovery service. This allows the kdump crash recovery service
complete without being preempted by traditional power fencing methods. 

fence_kdump_send

fence_kdump_send is a utility used to send messages that acknowledge that the 
node itself has entered the kdump crash recovery service. The fence_kdump_send
utility is typically run in the kdump kernel after a cluster node has 
encountered a kernel panic. Once the cluster node has entered the kdump crash 
recovery service, fence_kdump_send will periodically send messages to all 
cluster nodes. When the fence_kdump agent receives a valid message from the 
failed nodes, fencing is complete.

How to configure Pacemaker cluster environment:

If we want to use kdump in Pacemaker cluster environment, fence-agents-kdump
should be installed in every nodes in the cluster. You can achieve this via
the following command:

  # yum install -y fence-agents-kdump

Next is to add kdump_fence to the cluster. Assuming that the cluster consists 
of three nodes, they are node1, node2 and node3, and use Pacemaker to perform
resource management and pcs as cli configuration tool. 

With pcs it is easy to add a stonith resource to the cluster. For example, add
a stonith resource named mykdumpfence with fence type of fence_kdump via the 
following commands:
  
   # pcs stonith create mykdumpfence fence_kdump \
     pcmk_host_check=static-list pcmk_host_list="node1 node2 node3"
   # pcs stonith update mykdumpfence pcmk_monitor_action=metadata --force
   # pcs stonith update mykdumpfence pcmk_status_action=metadata --force
   # pcs stonith update mykdumpfence pcmk_reboot_action=off --force
   
Then enable stonith
   # pcs property set stonith-enabled=true

How to configure kdump:

Actually there are two ways how to configure fence_kdump support:

1) Pacemaker based clusters
     If you have successfully configured fence_kdump in Pacemaker, there is
     no need to add some special configuration in kdump. So please refer to
     Kexec-Kdump-howto file for more information.

2) Generic clusters
     For other types of clusters there are two configuration options in
     kdump.conf which enables fence_kdump support:

       fence_kdump_nodes <node(s)>
            Contains list of cluster node(s) separated by space to send
            fence_kdump notification to (this option is mandatory to enable
            fence_kdump)

       fence_kdump_args <arg(s)>
            Command line arguments for fence_kdump_send (it can contain
            all valid arguments except hosts to send notification to)

     These options will most probably be configured by your cluster software,
     so please refer to your cluster documentation how to enable fence_kdump
     support.

Please be aware that these two ways cannot be combined and 2) has precedence
over 1). It means that if fence_kdump is configured using fence_kdump_nodes
and fence_kdump_args options in kdump.conf, Pacemaker configuration is not
used even if it exists.