server 2008 R2 quorum disk necessary?

I am running Windows 2008 R2 and want to cluster two physical servers with a SAN for storage.

I read the following from: http://technet.microsoft.com/en-us/library/ff182359(WS.10).aspx

"The disk witness is a disk in the cluster storage that is designated to hold a copy of the cluster configuration database. A disk witness is part of some but not all quorum configurations, and it is recommended for most clusters with an even number of nodes."

I have not been able to get any meaningful detail on  the second sentence.

Do I need a separate physical disk on my SAN if I want to setup failover clustering?

I've seen references to quorum disk and witness disk in this context? Are they the same thing?

I've also seen references to "file share witness' as an alternative to a quorum or witness disk. What exactly is this? Does this substitute in some way for the quorum/witness disk?  

Is it that I ultimately need at least one of these alternatives to have a cluster?

I did set up a cluster to the point where everything seemed to be working but I do not recall ever dedicating a disk to be a quorum  or a disk witness or a file share witness or anything else - there was one disk in the SAN and all I recall doing was formatting it.
lineonecorpAsked:
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Jason WatkinsIT Project LeaderCommented:
The quorum resource holds the shared configuration, and data to be served on the cluster. SQL databases, Exchange 2003/2007 mail clusters are good examples of what would use a quorum resource. Any node can participate in the cluster, as long as it can access the data and the configuration details.

Quote...

"The witness disk is a disk in the cluster storage that is designated to hold a copy of the cluster configuration database. (A witness disk is part of some, not all, quorum configurations.)"

http://technet.microsoft.com/en-us/library/cc771509%28WS.10%29.aspx
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lineonecorpAuthor Commented:
Thanks for the link but I have read it and several others before posting here. The issue of quorum disk is still not clear to me however. I'll pose my questions a little differently in point form so perhaps you cold address each point separately the answer would be clearer to me.

Question 1:
Do I need a minimum of two physical disks in my SAN for clustering in 2008 R2 with one of those disks reserved for the Quorum disk? It seems that is not the case - it may be the recommended option - but it is not an absolute requirement.  For purposes of this question I am not interested in recommended or best practice - just what is possible.


Question 2:

If I don't use a quorum disk do I then need a 'file share witness' and what exactly is that and how is it configured? Again I am not interested in recommended or best practice, I am just looking for what is absolutely necessary.  

Question 3:

Can I create a cluster without either a quorum disk or a file share witness regardless of whether it's recommended or best practice?  

Question 4:

Just for terminology purposes is Witness Disk the same as Quorum disk?

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msmamjiCommented:
Before answering your questions, I would add here something that I will build upon in the answers.

In Windows Server 2008, in failover cluster, quorum defines consensus whether enough cluster members are present (or more appropriately available) to provide services.

Quorum is based on votes, in which nodes, file shares, or a shared disk have a vote, depending on the mode you selected to create the cluster.

Now, you have four types in Windows Server 2008
a) Node Majority: Only nodes in the cluster have a vote.  Quorum is maintained when more then one half the votes are available. So understandibly, you need to have odd number of nodes to make this work.

b) Node and Disk Majority:The nodes in the cluster and a witness disk have a vote. You need more then one half the votes for quorum to be attained. This works for even number of nodes and one witness disk.

c) Node and File Share Majority: The nodes in the cluster and a witness file share have a vote. You need more then one half votes to achieve quorum. This works with even number of nodes and one file share.

d) No Majority(Disk Only): Only Quorum-shared disk has a vote. This works for any number of nodes. Since Quorum disk rules, it decides how has the cluster right now.

Now for your question:
1) It depends. If you select a, you don't need any disk at all to setup a working cluster. If you select b, you need even number of nodes and one disk on SAN. Select c, and you need even number of servers and one witness disk. Select d, you need one disk on SAN and any number of nodes.

2) I guess the above response answers the question, if you don't want to use quorum disk, you can opt for witness disk or witness share or none. You Juste need to make sure you have provide enough votes to achieve majority. I you have even nodes, you need a third vote, be it witness disk, witness share or another node.
This link will guide you to setup a failover cluster with witness disk
http://blogs.technet.com/josebda/archive/2008/07/16/failover-clustering-for-hyper-v-with-file-server-storage.aspx

3)Response to 1 and 2 answer the question. Yes, you can, setting a cluster w/o witness disk or share. You need odd number of nodes to maintain quorum.

With the exception of d, all quorum mechanism work on majority vote model. lets take Node and disk Majority example with 4 nodes and one witness disk (Maximum 5 votes)


Node1      Node2      Node3      Node4      Witness Disk      Cluster Status      
Online      Online      Online      Online      Online            Online
Online      Online      Offline      Online      Offline            Online
Online      Online      Offline      Offline      Online            Online
Online      Online      Offline      Offline      Offline            Offline
Online      Offline      Offline      Offline      Online            Offline
Offline      Offline      Offline      Offline      Online            Offline
Offline      Offline      Offline      Offline      Offline            Offline


4) No. Not exactly, the Quorum disk holds all information about the cluster and has absolute authority. The disk in known as quorum disk, if it has those characteristic. It knows and decides who is owner for what cluster resourses and servises. If you lose a quorum disk, you lose the cluster. (This is what we had back in the days of WS03 and was known as server clusters.)
The Witness Disk is holds the copy of the info for the cluster but only contributes towards voting. Lets see if this makes more sense now

"The disk witness is a disk in the cluster storage that is designated to hold a copy of the cluster configuration database. A disk witness is part of some but not all quorum configurations, and it is recommended for most clusters with an even number of nodes."

Hope that made sense.

Regards,
Shahid
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lineonecorpAuthor Commented:
Thanks very much for a very thorough answer. Let me read through - it's a bit to absorb - and then get back to you if I have any questions. Thanks again for the work.
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lineonecorpAuthor Commented:
Still reading and researching. Should either ask more or close question in a few days.
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