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2.2. The Current State of the Cluster

Before one starts to configure a cluster, it is worth explaining how to view the finished product. For this purpose we have created the crm_mon utility that will display the current state of an active cluster. It can show the cluster status by node or by resource and can be used in either single-shot or dynamically-updating mode. There are also modes for displaying a list of the operations performed (grouped by node and resource) as well as information about failures.
Using this tool, you can examine the state of the cluster for irregularities and see how it responds when you cause or simulate failures.
Details on all the available options can be obtained using the crm_mon --help command. 
  # crm_mon
  ============
  Last updated: Fri Nov 23 15:26:13 2007
  Current DC: sles-3 (2298606a-6a8c-499a-9d25-76242f7006ec)
  3 Nodes configured.
  5 Resources configured.
  ============

  Node: sles-1 (1186dc9a-324d-425a-966e-d757e693dc86): online
      192.168.100.181    (heartbeat::ocf:IPaddr):    Started sles-1
      192.168.100.182    (heartbeat:IPaddr):        Started sles-1
      192.168.100.183    (heartbeat::ocf:IPaddr):    Started sles-1
      rsc_sles-1    (heartbeat::ocf:IPaddr):    Started sles-1
      child_DoFencing:2    (stonith:external/vmware):    Started sles-1
  Node: sles-2 (02fb99a8-e30e-482f-b3ad-0fb3ce27d088): standby
  Node: sles-3 (2298606a-6a8c-499a-9d25-76242f7006ec): online
      rsc_sles-2    (heartbeat::ocf:IPaddr):    Started sles-3
      rsc_sles-3    (heartbeat::ocf:IPaddr):    Started sles-3
      child_DoFencing:0    (stonith:external/vmware):    Started sles-3
Figure 2.1. Sample output from crm_mon

  # crm_mon -n
  ============
  Last updated: Fri Nov 23 15:26:13 2007
  Current DC: sles-3 (2298606a-6a8c-499a-9d25-76242f7006ec)
  3 Nodes configured.
  5 Resources configured.
  ============

  Node: sles-1 (1186dc9a-324d-425a-966e-d757e693dc86): online
  Node: sles-2 (02fb99a8-e30e-482f-b3ad-0fb3ce27d088): standby
  Node: sles-3 (2298606a-6a8c-499a-9d25-76242f7006ec): online

  Resource Group: group-1
    192.168.100.181    (heartbeat::ocf:IPaddr):    Started sles-1
    192.168.100.182    (heartbeat:IPaddr):        Started sles-1
    192.168.100.183    (heartbeat::ocf:IPaddr):    Started sles-1
  rsc_sles-1    (heartbeat::ocf:IPaddr):    Started sles-1
  rsc_sles-2    (heartbeat::ocf:IPaddr):    Started sles-3
  rsc_sles-3    (heartbeat::ocf:IPaddr):    Started sles-3
  Clone Set: DoFencing
    child_DoFencing:0    (stonith:external/vmware):    Started sles-3
    child_DoFencing:1    (stonith:external/vmware):    Stopped
    child_DoFencing:2    (stonith:external/vmware):    Started sles-1
Figure 2.2. Sample output from crm_mon -n

The DC (Designated Controller) node is where all the decisions are made and if the current DC fails a new one is elected from the remaining cluster nodes. The choice of DC is of no significance to an administrator beyond the fact that its logs will generally be more interesting.