MCSG Issue

[ramkumar]

Hi folks,
Can any body tell whether Cluster will form properly when i am running cmruncl and cluster lock disk was not visible for both nodes.

regards
Ramkumar.G

[Luk Vandenbussche]

Hi,

I don't think this will work.
You will receive several errors in the syslog.log file

[melvyn burnard]

The cluster will form, but if the cl;uster lock disc is missing, then you will see errors logged in the syslog.log every hour (polling interval to check the disc), and if oner node were to crash, the other would TOC.

[Geoff Wild]

No - only time cluster will function without a lock disk (or quorum server) is when it is already running. Reformations will not work - cluster startup will fail.

Rgds...Geoff

[Geoff Wild]

Ops - ignore my previous post - a 2 node cluster will reform - from Managing Serviceguard:

"Although a cluster quorum of more than 50% is generally required, exactly 50% of the previously running nodes may re-form as a new cluster provided that the other 50% of the previously running nodes do not also re-form. This is guaranteed by the use of a tie-breaker to choose between the two equal-sized node groups, allowing one group to form the cluster and forcing the other group to shut down. This tie-breaker is known as a cluster lock. The cluster lock is implemented either by means of a lock disk or a quorum server.

The cluster lock is used as a tie-breaker only for situations in which a running cluster fails and, as Serviceguard attempts to form a new cluster, the cluster is split into two sub-clusters of equal size. Each sub-cluster will attempt to acquire the cluster lock. The sub-cluster which gets the cluster lock will form the new cluster, preventing the possibility of two sub-clusters running at the same time. If the two sub-clusters are of unequal size, the sub-cluster with greater than 50% of the nodes will form the new cluster, and the cluster lock is not used.

If you have a two-node cluster, you are required to configure a cluster lock. If communications are lost between these two nodes, the node that obtains the cluster lock will take over the cluster and the other node will halt or perform a TOC. Without a cluster lock, a failure of either node in the cluster will cause the other node, and therefore the cluster, to halt. Note also that if the cluster lock fails during an attempt to acquire it, the cluster will halt. "

Basically - as long as there are 50% of the nodes up - it will reform.

Rgds...Geoff

[Sundar_7]

Hey Ram,

cmruncl should work,provided both the nodes are up. cmcld periodically checks to make sure cluster lock is accessible. If not, error messages will be logged in syslog.log.

If you want to form a cluster only with specific node(s), then -n option can be used with cmruncl.

Thanks,

Sundar.

[Sheriff Andy]

Ram,

As long as there is a heartbeat between the 2 nodes, you should be able to bring up your cluster. Be advised though, if you reboot one of your nodes without doing a cmhaltnode 1st, there is a good chance that you will cause the other node to TOC. This happened in our workplace (until we got the lock disk issue resolved).

Andy

[Bharat Katkar]

Hi Ramkumar,

This is exactly called as split brain scenario. If the cluster lock disk is not available then both the node will form a cluster and also if you are trying to run same package from both the node, the risk of data corruption is there.

Regards,