topic Re: Cluster Lock is required in Operating System - HP-UX

Cluster Lock is required

P Arumugavel — Tue, 16 Nov 2010 11:12:44 GMT

Hi experts,

For a three node SG cluster, is cluster lock required?

Re: Cluster Lock is required

rariasn — Tue, 16 Nov 2010 11:48:14 GMT

Hi:

Lock Requirements

A one-node cluster does not require a cluster lock.

A two-node cluster

requires a cluster lock.

In clusters larger than three nodes, a cluster lock is strongly recommended.

If you have a cluster with more than four
nodes, use a quorum server; a cluster lock disk is not allowed for clusters of that size.

rgs,

Re: Cluster Lock is required

Jayakrishnan G Naik — Tue, 16 Nov 2010 11:50:07 GMT

Hi Vel-UX

Cluster lock is recommended mainly for 2 node clusters and I have heard, for 3 node clusters as well. For 3 or more nodes quorum server will be a better solution.

The cluster lock avoids the split brain syndrom, and in a two node cluster, the quorum is lost when a node fails so the to maintain quorum(more than 50%) another vote is held by the quorum disk.
when we discuss about 3 node clusters I have seen people turning to lock disk rather than a quorum server, and the reason given is that, in case of a node failure -say hardware failure or a planned down time on one node, the cluster reduces to a two nodes . The chance of split brain or lose of quorum due to another failure in running nodes remains and that can be avoided if 3 node cluster have lock disk.

Thanks & regards
Jayakrishnan G Naik

Re: Cluster Lock is required

Stephen Doud — Tue, 16 Nov 2010 12:34:38 GMT

It took me longer to write this response than to find the answer for you.

Use a browser to navigate to http://docs.hp.com/en/ha.html -> HP Serviceguard -> User Guide -> "Managing Serviceguard Eighteenth Edition, September 2010" manual (at http://bizsupport2.austin.hp.com/bc/docs/support/SupportManual/c02437444/c02437444.pdf).

Then search the PDF document with keyword string "cluster lock disk".
The answer is on page 63

Happy Fishing :)

Re: Cluster Lock is required

Emil Velez — Tue, 16 Nov 2010 21:39:15 GMT

you can get away without a cluster lock for a 3 node cluster if and only if when you loose a node you will not loose another node.

if you go to a 2 node cluster and loose the next node both nodes will panic without quorum.

IF you have a 3 node cluster and if you loose a node and you know that you will fix the node and return it to service before loosing another node then you do not need a cluster lock.

IF you have a LVM volume group that is accessable by all nodes and that a package uses why not use a cluster lock disk. This way no additional disks or systems are necessary. Nothing wrong with cluster lock. If it fails fix it!!!!

Re: Cluster Lock is required

P Arumugavel — Fri, 19 Nov 2010 05:15:44 GMT

Hi Thanks...

For an example, if I use a three node cluster and the package is running on Node A.
Node B and Node C are adoptive node respectively. Lock disk is made in one of shared VG. Now let me understand that what are all situations that would make the use of Lock disk?

Re: Cluster Lock is required

Torsten. — Fri, 19 Nov 2010 06:57:06 GMT

All situation where a "split brain" can occur (e.g. if 1 node is down, you have a 2 node cluster; if something bad happens now - lost communication for example - you need the lock).

Re: Cluster Lock is required

P Arumugavel — Fri, 19 Nov 2010 07:28:35 GMT

Hi Torsten,

OK. In 2 node cluster, if Communication fails, split brain can occur (50-50% available nodes). Here Lock disk will be acquired by one of the node. But in a three node cluster, what is the cahance there split brain can occur? If my first node fails then package will fail over to the next adoptive node...

Re: Cluster Lock is required

melvyn burnard — Fri, 19 Nov 2010 08:14:56 GMT

and what if you have one of the 3 nodes "out of service" as far as cluster activities are concerned, perhaps fo patching?
If you then get a failure on one of the two remaining nodes, then ou have a 50-50% split again.
Recommendation is to have some form of arbitration, no matter what size cluster you have

Re: Cluster Lock is required

Torsten. — Fri, 19 Nov 2010 11:59:43 GMT

This exactly what I was talking about. 2 nodes active, 1 intensionally down for whatever reason.

And IMHO in rare cases you can even have a situation of 3 splitted brains.

Re: Cluster Lock is required

P Arumugavel — Mon, 22 Nov 2010 16:43:49 GMT

Thanks to all.

With three or four nodes, a cluster lock is recommended but not required. For three nodes, there would never be a situation where the nodes are divided 50/50. However, if a node is pulled out of the cluster for maintenance purposes, then the cluster lock would be used should one of the remaining two nodes in the cluster fail....