Re: What is the Difference between Shadow and Mirrored disk?

From: Roy Omond (Roy.Omond_at_BlueBubble.UK.Com)
Date: 03/03/05 

Date: Thu, 03 Mar 2005 12:36:12 +0000

Phillip Helbig wrote:

> In article <38lkqfF5os0vvU1@individual.net>, Roy Omond
> <Roy.Omond@BlueBubble.UK.Com> writes:
>
>>Sorry to harp on about this, but since it's coming from an HP address,
>>I'd just like to add the comment that using a *local* quorum disk as
>>suggested is not only unnecessary, but actually *increases* the
>>probability of losing the cluster.
>>
>>Node A (1 vote) + local quorum disk (1 vote)
>>Node B (1 vote)
>>
>>Node B goes down ...
>>
>>Now you have introduced an unnecessary probability of the cluster
>>being lost, equal to the probability of the quorum disk "disappearing".
>
> In the situation above, if the quorum disk disappears, then there are
> still 2 out of 3 votes there, so the cluster will survive. The reason
> this is pointless, though, is since if node A goes down, so does the
> disk, so the above is equivalent to giving node A two votes and node B 1
> vote.

No Phillip, you've not read my posting with sufficient care.

Note the line "Node B goes down ...", and re-read.

And, no it's not equivalent. The quorum disk *increases* the
probability of cluster failure.

> With a two-node cluster without a "proper" quorum disk (which is really
> a three-node cluster with the disk taking the place of the third node),
> there is NO WAY to have a real cluster, whether or not a "local quorum
> disk" is used and however the votes are distributed. Either you have a
> 50-50 situation: cluster freezes if either node goes down, or you have a
> master-slave situation, where one node must ALWAYS be up for the cluster
> to be up and it doesn't matter what the other one does.

Unless you've got separate SYSUAFs, Rightslists etc. (not a good idea,
IMHO), then the master-slave is much better than the other method.

Hint: MVTIMEOUT

> I really fail to see why people these days even bother with less than
> optimal solutions. Just get a cheap VAX and use it as a quorum node;
> problem solved.

Agreed. Even better, get a cheap (preferably free) ES47 or GS1280 :-)

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