Cluster Quorum Options.
What does quorum mean anyway?
Well, quorum, in our context for server high availability,
refers to the number of failures required to take a cluster offline.
A node or a Windows Server box that's participating in a failover
cluster is going to have a vote, and then depending upon whether
you have an even or odd number of votes,
you can bring in a witness to provide a tiebreaker vote.
For example, in my lab environment here,
I have a two‑node cluster. Ideally, you should have an odd
number of nodes in your cluster because the votes are
counted such that the majority wins.
So if over 50% of the vote casters in a failover cluster
report that they're online and available,
then that would mean that the cluster continues to operate.
But what if you're in a situation with an even number of voters
where you've got one down and one up? Then you have a tie, and
this brings in the idea of a witness.
So Microsoft's guidance here is that your clusters should always have an odd
number of total voters so you don't run into a tie situation.
For example,
with a four‑node cluster, three voters must be online in order to avoid a tie.
Now let's understand the specific configuration options available to us as
Windows Server administrators with regard to quorum.
If you do want to bring in a witness as a tie breaking
vote, your options are threefold.
The first is the disk witness,
This is literally a disk that's accessible to all nodes in
the cluster, and it can cast a vote itself.
This is not a supported witness type when you're using S2D, or Storage Spaces
Direct, only when you're using a storage area network array. Microsoft
recommends considering disk witness if you are using a non‑S2D shared storage
solution and also your within a single site.
This is because the cluster nodes all need to be able to
see the disk. For multi‑site clusters,
you can do a file share witness. This is an SMB, or Server
Message Block, share on a cluster node.
And this is best because that share should be accessible or
viewable across your multi‑site architecture.
And then lastly, you've got the cloud witness, and this is a shared folder,
an SMB share, in an Azure storage account.
And this is good when your cluster nodes can go either over the internet or
over a private endpoint link or over a site‑to‑site VPN or over an ExpressRoute circuit in order to reach that cloud witness.