HP Serviceguard Quorum Server

[Shivkumar]

After installing HP Serviceguard Quorum Server; how to use it ? i mean do we need to launch a browser where we can see the cluster nodes ?

Thanks,
Shiv

[Mel Burslan · Accepted solution]

Quorumn server is not something that gets into the cluster formation. It is only there as a tie-breaker in the case two nodes can not communicate one another. Whichever node contacts the quorum server first, gets to be the cluster owner. The other one, more than likely, will TC itself and go down. This prevents the split brain syndrome of the cluster. After the cluster gets formed, the need for the QS will be over and it sits on the side until the next cluster reform.

How you see the nodes of the cluster is basically upto you. I never used a graphical viewer, but have seen our HP consultant use it. It did not look like a browser to me. I think it was what-so-called SGM (service guard manager) as he was changing the cluster configuration by drag and drop action. More info on the SGM can be found here:
http://docs.hp.com/en/B8325-90036/ch01s03.html
as well as a link to a full PDF doc.

[melvyn burnard]

Quorum Server is a product (free) that you install on a server outside of the cluster.
You then configure it to run as a service, spawned by init, in /etc/inittab.
When you then configure a cluster, you supply the hostname or ip address of the quorum server in the cmquerycl command line using the -q option.
Instructions on how to configure QS are found on pages 196-197 of:
http://docs.hp.com/en/B3936-90079/B3936-90079.pdf

[Ranjith_5]

Hi Shiva,


The quorum server is an alternate form of cluster lock that uses a server program running on a separate system for tie-breaking. Should equal sized groups of nodes become separated from each other, the quorum server allows one group to achieve quorum and form the cluster, while the other group is denied quorum and cannot start a cluster.

The quorum server process runs on a machine outside of the cluster for which it is providing quorum services. The quorum server listens to connection requests from the ServiceGuard nodes on port # 1238. The server maintains a special area in memory for each cluster, and when a node obtains the cluster lock, this area is marked so that other nodes will recognize the lock as "taken."

In general, the algorithm for cluster re-formation requires a cluster quorum of a strict majority (that is, more than 50%) of the nodes previously running. If both halves (exactly 50%) of a previously running cluster were allowed to re-form, there would be a split-brain situation in which two instances of the same cluster were running. In a split-brain scenario, different incarnations of an application could end up simultaneously accessing the same disks. One incarnation might well be initiating recovery activity while the other is modifying the state of the disks. ServiceGuard's quorum requirement is designed to prevent a split-brain situation

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.


HTH

Regards,
Syam

[Shivkumar]

Can anyone explain below 3 points:-

1) Quorum Server is running on a separate box. I just installed on a machine.

I have defined 8 (server names)
nodes in the config file:
/etc/cmcluster/qs_authfile

4 nodes are defined in 2 different network zone.

I noticed that the Quorum server is running with a process name "qsc".
Now my question is using this piece of software how to manage cluster nodes.

2) What is meaning of disk or cluster lock ?

3) When we use package ?

Thanks,
Shiv

[melvyn burnard]

1)I noticed that the Quorum server is running with a process name "qsc".
Now my question is using this piece of software how to manage cluster nodes.

You do not use Quorum Server to manage any nodes, it is used BY the SG cluster nodes for tie breaking, the same as a cluster lock disc.

2) What is meaning of disk or cluster lock ?

This is a method where the surviving modes in an SG cluster try to reform, and if there is EXACTLY 50% of the nodes available, by SG ruules this is not enough, we need to have a "lock" or tie break mechanism to give us more than the 50%. This is what a cluster lock disc or quorum server do for the cluster.



3) When we use package ?
What do you mean ??


I suggest you take a read of this :
http://docs.hp.com/en/B3936-90078/B3936-90078.pdf

And also consider taking a Serviceguard class.

[Kent Ostby]

Shivkumar --

Please take a look at the certified document:

UMCSGKBRC00016827 (How to configure a quorum server)

http://www2.itrc.hp.com/service/cki/search.do?category=c0&prevQueryString=&mode=id&searchString=UMCSGKBRC00016827&searchCrit=allwords&docType=EngineerNotes&dateRange=all&search.x=22&search.y=6

and

UMCSGKBRC00012642 (difference between lock disk and a quorum server)

http://www2.itrc.hp.com/service/cki/search.do?category=c0&prevQueryString=&mode=id&searchString=UMCSGKBRC00012642&searchCrit=allwords&docType=EngineerNotes&dateRange=all&search.x=31&search.y=11

Best regards,

Oz

[Mel Burslan]

Shiv,

As I have explained im y fisrt post, you do not manage clusters by quorum server. Quorum server is only used for tie-breaking.

Managing a cluster is done using cluster administration commands from the command line, such as:

cmviewcl
cmrunpkg
etc.

or using SGM on your desktop (windows) machine.

quorum server does not have any role on a running cluster and the qsc process is a very low overhead process usually does nothing but listen to a port for incoming communication.

[Ranjith_5]

Hi Shivakumar,

Answering to your questions 1&2,

The cluster lock is used as a tiebreaker for situations in which a running cluster fails, and then two equal-sized sub-clusters are both trying to form a new cluster. The cluster lock may be configured using either a lock disk or a quorum server.

You can use either the quorum server or the lock disk as a cluster lock but not both in the same cluster. Consider the following when configuring a cluster.For a two-node cluster, you must use a cluster lock. For a cluster of three or four nodes, a cluster lock is strongly recommended. For a cluster of more than four nodes, a cluster lock is recommended. If you decide to configure a lock for a cluster of more than four nodes, it must be a quorum server.


Answer to Q3. For a easy understanding, in an MC Service guard cluster your applications/data bases are configured as packages. Packages control the starup & shutdown of the dabase/application as well. Packages give redundancy to your applications, by switching the package to another node in a cluster when there is a hardware failure.

Regards,
Syam

[Arunvijai_4]

http://docs.hp.com/en/B3936-90079/ch05s02.html
http://docs.hp.com/en/B8467-90001/ch01s04.html

-Arun