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Changing votes in a cluster without a cluster reboot

 

Changing votes in a cluster without a cluster reboot

I had a cluster of 4 Alpha VMS servers, each with 1 votes & a quorum disk with 3 votes; so that I could have at least 1 server up. (Quorum = 4 - either all 4 nodes or at least 1 nodes and the quorum disk.)
One old node has failed and wont be replaced. Now I need the quorum disk votes to maintain quorum. This is a 7x24 system and I can't reboot the cluster to adjust votes, expected votes, etc. easily. I can reboot one of the nodes without an application shutdown. My thought was to give that node 2 votes instead of 1. Then I'd have quorum with the 3 nodes (1 vote + 1 vote + 2 votes).
Because of, as yet diagnosed problems, the quorum watcher keeps loosing connection to the quorum disk (NAS array) and the cluster hangs for a few seconds.
Giving one node 2 votes doesn't fix the problem of losing connection to the quorum disk, but I'm hoping to mitigate the cluster hangs until the connection problem can be diagnosed and resolved.

I'm just looking at a sanity check on the proposed change so that it doesn't result in a split cluster.
Expected votes still = 7
Quorum disk votes still = 3
2 nodes with votes still = 1
1 node with votes = 2 (modparams changed and node rebooted)
Quorum still 4 (3 nodes up; or at least 1 node with the quorum disk votes).

When I can reboot the cluster, I'll adjust everything to be more relevant to the configuration.
13 REPLIES 13
Hoff
Honored Contributor

Re: Changing votes in a cluster without a cluster reboot

Adding an extra vote here will allow the existing hosts to ride over a problem with the quorum disk, and would avoid the I/O polling inherently required when the votes for the quorum disk need be counted toward quorum.

The delay for a cluster transition would be 3*QDSKINTERVAL when the votes from the quorum disk are required.

Given this environment could well involve a network problem, acquiring host-based votes without depending on the quorum disk may not be a sufficient salve for the instabilities. Clusters tend to be unstable when the network is unstable, and partitions (and hangs) can arise.
P Muralidhar Kini
Honored Contributor

Re: Changing votes in a cluster without a cluster reboot

Hi Carleen,

In the new setup you would have 3 nodes and a Quorum disk.
Quorum disk is generally recommended in case of a 2 node cluster.

In the new setup, you can have 1 vote each for the 3 nodes and do away with
the Quorum disk.
In this case,
Each node would have Vote = 1
Quorum = 2
Hence, If any two of the nodes are up then the cluster would be up.

Even in the new setup, do you want a configuration such that,
even if one node is up, you want the cluster to be up ?

Regards,
Murali
Let There Be Rock - AC/DC
P Muralidhar Kini
Honored Contributor

Re: Changing votes in a cluster without a cluster reboot

Hi Carleen,

The following link talks about OpenVMS cluster concepts.
http://h71000.www7.hp.com/doc/731final/4477/4477pro_002.html

This talks about Votes/Quorum/Quorum disk/Quorum disk watcher
and also rules for Specifying Quorum.

Regards,
Murali
Let There Be Rock - AC/DC
Hoff
Honored Contributor

Re: Changing votes in a cluster without a cluster reboot

Murali, quorum disks are common in some of these cluster configurations with rather more than two hosts.

In particular, for cluster configurations that are intended to survive the loss or downing of comparatively large numbers of hosts. A three node cluster that can survive with two of the nodes down, for instance, and as parallels this case.

With hardware RAID underneath the quorum disk and with comparatively short polling timers, these configurations do work nicely for the specific requirements.

With a short timer value, the cluster will have three (polling at 1 second intervals) to six (at 2) to nine (at 3) seconds for a transition if/when the votes from the quorum disk are needed.
P Muralidhar Kini
Honored Contributor

Re: Changing votes in a cluster without a cluster reboot

Hi Hoff,

>> A three node cluster that can survive with two of the nodes down, for
>> instance, and as parallels this case.
Yes, i get your point. You have give a good example as to why quorum disk
would be required in general (for any set of nodes in a cluster).
And this is exactly the reason why carleen is using the quorum disk in the
cluster setup.

Regards,
Murali
Let There Be Rock - AC/DC