cluster lock on a cluster with 2 nodes

[ANDRE David]

Hi,

I have a cluster lock on a cluster with 2 nodes.

the Vg is vgcla
the path is c4t2d0

This vg is dedicated only on cluster lock.

my question is :

This vg must be up ( vgchange -a e vgcla ) or down ( vgchange -a n vgcla ) in running mode normal?

Thanks.

[G. Vrijhoeven]

Hi,

The answer is simple for MC/SG that does not matter. If you can spare a disk you can leave it unused. Iy You need the diskspace you are able to use the disk ( vg ) and put data on it. This does not matter for the lock meganism.

For more info check:

http://docs.hp.com/cgi-bin/fsearch/framedisplay?top=/hpux/onlinedocs/B3936-90024/B3936-90024_top.html&con=/hpux/onlinedocs/B3936-90024/00/00/15-con.html&toc=/hpux/onlinedocs/B3936-90024/00/00/15-toc.html&searchterms=lock&queryid=20040126-031242

It states:
Use of the Cluster Lock

The cluster lock is a disk area located in a volume group that is shared by all nodes in the cluster. The cluster lock volume group and physical volume names are identified in the cluster configuration file. The cluster lock is used as a tie-breaker only for situations in which a running cluster fails and, as MC/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 the cluster lock. If communications are lost between these two nodes, the node with the cluster lock will take over the cluster and the other node will shut down. 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.

You can choose between two cluster lock options -- a single or dual cluster lock -- based on the kind of high availability configuration you are building. A single cluster lock is recommended where possible. With both single and dual locks, however, it is important that the cluster lock disk be available even if one node loses power; thus, the choice of a lock configuration depends partly on the number of power circuits available. Regardless of your choice, all nodes in the cluster must have access to the cluster lock to maintain high availability.

If you have a cluster with more than 4 nodes, a cluster lock is not allowed.


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Plan ahead for the future growth of the cluster. If the cluster will grow to more than 4 nodes, you should either not configure a cluster lock or else plan on taking down the cluster while the fifth node is added so the cluster lock can be removed.

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Single Cluster Lock

It is recommended that you use a single cluster lock. A single cluster lock should be configured on a power circuit separate from that of any node in the cluster. For example, it is highly recommended to use three power circuits for a two-node cluster, with a single, separately powered disk for the cluster lock. For two-node clusters, this single lock disk may not share a power circuit with either node, and it must be an external disk. For three or four node clusters, the disk should not share a power circuit with 50% or more of the nodes.

Dual Cluster Lock

If you are using disks that are internally mounted in the same cabinet as the cluster nodes, then a single lock disk would be a single point of failure in this type of cluster, since the loss of power to the node that has the lock disk in its cabinet would also render the cluster lock unavailable. In this case only, a dual cluster lock, with two separately powered cluster disks, should be used to eliminate the lock disk as a single point of failure. For a dual cluster lock, the disks must not share either a power circuit or a node chassis with one another. In this case, if there is a power failure affecting one node and disk, the other node and disk remain available, so cluster re-formation can take place on the remaining node.

No Cluster Lock

Normally, you should not configure a cluster of three or fewer nodes without a cluster lock. In two-node clusters, a cluster lock is required. You may consider using no cluster lock with configurations of three or more nodes, although the decision should be affected by the fact that any cluster may require tie-breaking. For example, if one node in a three-node cluster is removed for maintenance, the cluster reforms as a two-node cluster. If a tie-breaking scenario later occurs due to a node or communication failure, the entire cluster will become unavailable.

In a cluster with four or more nodes, you do not need a cluster lock since the chance of the cluster being split into two halves of equal size is very small. Cluster locks are not allowed in clusters of more than four nodes. However, be sure to configure your cluster to prevent the failure of exactly half the nodes at one time. For example, make sure there is no potential single point of failure such as a single LAN between equal numbers of nodes, or that you don't have exactly half of the nodes on a single power circuit.

Backing Up Cluster Lock Information

After you configure the cluster and create the cluster lock volume group and physical volume, you should create a backup of the volume group configuration data on each lock volume group. Use the vgcfgbackup command for each lock volume group you have configured, and save the backup file in case the lock configuration must be restored to a new disk with the vgcfgrestore command following a disk failure.


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You must use the vgcfgbackup and vgcfgrestore commands to back up and restore the lock volume group configuration data regardless of whether you use SAM or HP-UX commands to create the lock volume group.

HTH,

Gideon

[melvyn burnard]

there is no requirement for the vg to be activated in normal running mode, but as the cluster lock disc is usually part of a shared data vg, then it normally is activated by the application package.
If you are lucky enough to have the spare disc availability to have a single disc vg for cluster locking, then this works as well.