Checking Corosync cluster membership

It’s simple and easy to get Pacemaker’s view of the status of members in a cluster – just invoke crm_mon. But what if you want to check on the cluster membership when Pacemaker is not running, or you want to make sure whether Corosync’s view of the cluster is identical to Pacemaker’s? Here’s how.

Checking ring status with corosync-cfgtool

The corosync-cfgtool utility displays the cluster connectivity status when invoked with the -s flag:

# corosync-cfgtool -s
Printing ring status.
Local node ID 303938909
RING ID 0
    id  = 10.0.1.1
    status  = ring 0 active with no faults
RING ID 1
    id  = 192.168.42.1
    status  = ring 1 active with no faults

The above is the status of two healthy rings; a failed ring (one affected by a network interruption, for example) would show a FAULTY status.

There’s a catch. In a two-node cluster, if both nodes were to start while all cluster communication links are down, then Corosync would form two memberships with healthy, one-member rings. Both of the nodes would show a ring status similar to the above, but your cluster still wouldn’t be communicating. So, you can’t rely on corosync-cfgtool -s alone. You must also check Corosync’s member list.

Querying the member list with corosync-cmapctl

We can examine Corosync’s cluster member list with the corosync-cmapctl command:

# corosync-cmapctl | grep member
runtime.totem.pg.mrp.srp.members.303938909.ip=r(0) ip(10.0.1.1) r(1) ip(192.168.42.1) 
runtime.totem.pg.mrp.srp.members.303938909.join_count=1
runtime.totem.pg.mrp.srp.members.303938909.status=joined
runtime.totem.pg.mrp.srp.members.320716125.ip=r(0) ip(10.0.1.2) r(1) ip(192.168.42.2) 
runtime.totem.pg.mrp.srp.members.320716125.join_count=1
runtime.totem.pg.mrp.srp.members.320716125.status=joined

In this example, we have two nodes (with node IDs 303938909 and 320716125). They are both configured to use two communication rings, r(0) and r(1), and both of them have successfully joined the cluster.

Note: In earlier Corosync releases (pre-2.0), the corosync-cmapctl tool was called corosync-objctl. Its command syntax for querying the member list was identical.

This article originally appeared on the hastexo.com website (now defunct).