In this post we will look at deploying the HP 3PAR quorum witness – this is a vital system when running arrays with Peer Persistence synchronous replication as it allows us to arbitrate in a split brain scenario.
First off what does it do and why do we care?
The quorum witness is a virtual machine appliance created by HP. Let us assume we have 2 datacenters (DCs) each with a 3PAR array and that these arrays are synchronously replicating to one another. We have VMware ESXi hosts presented to both arrays and we are leveraging ALUA to present active and standby paths. The witness should be deployed in a third site with good connectivity to both of the DCs. The 3PAR arrays constantly talk to this witness and it will act as an arbitrator if the links between DCs go down.
I’ve created some simple diagrams below to demonstrate this along with a few failure scenarios to consider and what happens.
Healthy System – normal function
Inter-DC Link Failure
Inter-DC and Single Array Failure
Quorum Witness Failure: Inter-DC Links and Arrays Healthy
Now it’s important to note that I am summarising and skimming over a lot of details and essentially giving a ‘mile high’ view of the setup and scenarios. The important part is to understand that when everything in your environment is running OK you won’t mind the witness working or not. That being said, when you do have an issue it’s vital the witness is in place to play it’s part.