What is lock disk in a cluster

[Vishu]

Hi experts,
As i am new to the Cluster environment, so please share some information on Lock disk in a MC/SG cluster.

Regards
Vishu

[Ivan Krastev]

See here - http://docs.hp.com/en/B3936-90117/ch03s02.html#babbgfdh

regards,
ivan

[S. Ney]

Hi this is from our setup:

In order for a cluster to reform after a node failure, a strict majority (more than 50%) of the nodes previously running must attempt to join the new cluster. Exactly 50% of the previously running nodes may reform as a new cluster provided that the other 50% do not also reform as a new
cluster. This will most often happen when a cluster contains two nodes, and heartbeat
communications between the two nodes are interrupted. Each node will assume that the other has failed, and will attempt to form a cluster by itself. Since only one cluster can be allowed to reform, a tie-breaker or cluster lock is needed. This cluster lock can be either a disk designated
for this purpose that is physically connected to all nodes in the cluster or another system acting as a Quorum Server.

In the case of a cluster lock disk, an area outside the normal data area is used. When two subclusters of equal size attempt to form, each sub-cluster will attempt to acquire the cluster lock.
The sub-cluster that successfully obtains the cluster lock will form the new cluster, while the other sub-cluster will be prevented from forming. A cluster lock disk cannot be used for a cluster of
more than four nodes.

A cluster locking mechanism is required for a two-node cluster, and is strongly recommended for a three- or four-node cluster. Note that in the case of a two-node cluster, if one node fails the
other must gain access to the cluster lock. If it cannot, Serviceguard will not be able to distinguish this condition from the condition where both nodes have lost contact with each other and each is trying to reform as a one-node cluster. Either (or both) nodes that are trying to create
a one-node cluster will be halted and rebooted.

Our service guard_cluster is configured to use a cluster lock disk. For this cluster, a volume group, vglock, was established to hold the cluster lock disk.

[Grayh]

The following example should give you some Idea about Lock disk and its usage

1. Halt the cluster
cmhaltcl -f
2. On all nodes:
rm /etc/cmcluster/*
3. On a two node cluster, enable at least one of the volume groups used for packages. This volume group will be used for the lock disk.
vgchange -c n ${vg}
vgchange -a y ${vg}
4. Generate a cluster configuration script.
1. cmquerycl -C ${config} -v -n ${node1} ... -n ${node#}
2. cmgetconf -c ${cluster} > ${config}
5. Edit the configuration file as appropriate
6. cmcheckconf -C ${config}
7. cmapplyconf -C ${config}

[piyush mathiya]

Hi Vishu,


Lock disk in cluster means, whenever your one node is not able to ping other node through heartbit cable, at that time other node will check the lock disk first, if it is already in use then it will not start the package again...

Like,
If your heartbit cable went down, so both the node will treat as, other node is down and it will tye to activate package. but if Disk lock is not into the picture, then both the node will start the package at the same time and data can be crash...
If you still having doubt, please let me know...

[jaivinder]

Lock disk is a important part of cluster. It is only used when there is a cluster reformation. Lock disk is suitable up to 4 node cluster,if you cluster is having more then 4 node then the quorum server is used. The lock disk is a shared disk by all the nodes of the cluster. It is the normal disk and it is only used when their is a cluster reformation, because while the cluster reformation the first node which will acquire the lock on the Lock disk will start the cluster manager.The Lock disk is very important because in a two node cluster if both the nodes start the cluster manager then there will be two cluster instances, and it is known as Split Brain syndrome. To stop the Split Brain syndrome in MC Servicegarud the lock disk is used. If the lock disk goes bad ,after replacing the Lock disk the cluster configuration binaries are also updated using the cmquarycl command.

[Johnson Punniyalingam]

Hi Vishu,

>>>so please share some information on Lock disk in a MC/SG cluster.<<<<<

Cluster lock disk

The Cluster lock is a disk located in a volume group shared by all the nodes in the cluster . The cluster lock volume group and the pshyical volumer are identified in the Cluster configuration file,

The cluster lock is used as a tiebraker only for the situation in which the running cluster fails and atempt to refrom the cluster, the cluster is split init two subcluster of equal size , Each sub cluster will attempt to acquire the cluster lock.

the sub cksuter that gets the cluster lock will form the cluster , The subcluster that gets the cluster lock qill form the new cluster , The Sub cluster that does not get the cluster lock will panic in order to allow existing application to failover to

the new cluster which has been formed

[Md. Farhan A Azam]

Hi,

http://docs.hp.com/en/B3936-90122/ch05s02.html#babdibda

Plz refer this document.