ESX Virtualization

VMware ESXi, vSphere, VMware Backup, Hyper-V... how-to, videos....

Cluster with 2-Nodes only – How about quorum?

By | Last Updated:

Shares

If you're thinking about 2-Node cluster configuration, one of the challenges might be the question might be on how split-brain scenarios are handled? To remind you, a split-brain scenario is when your cluster has a node failure or there is a network failure between two nodes, and then both systems are back online again. Which one has the most recent data? Who decides? Usually, there is a quorum involved, a third system storing some elements about the connections, date and hour of access etc. Quorum is here and exists to actually avoid Split-Brain and prevents corruption on disks.

The thing is that you need that third system for storing those elements, system that's called Witness for VMware vSAN or StarWind VSAN. For VMware vSAN 2-Nodes, since v7 U1, there is a capability to use a shared witness appliance instance across multiple 2-Node deployments. But still, this Witness has to run somewhere, you must keep network connection between the node and the Witness, etc.

Can I run a 2-Node cluster without Quorum?

Yes you can, with StarWind VSAN configured to use a Heartbeat failover policy. You can then select the Synchronous “Two-Way” replication.

If one node fails, VMware vSphere HA will restart VMs on the remaining node. (Note: You'll need at least vSphere Essentials Plus for this to work, Essentials only does not have HA).

With heartbeat failover strategy, the storage cluster will continue working with only one StarWind node available.

After the node is repaired and come back online, the synchronization of data between the two nodes is done by StarWind automatically. (Note: you have to configure auto-start VM for StarWind appliance on each host so the StarWind appliance is actually started automatically too.).

The UI from the lab

However, if you do have 3rd system already, within your environment, you can obviously configure it as a Witness node. And this is done via the other strategy present with StarWind VSAN which is called a Node Majority Failover Strategy. 

This strategy allows synchronization connection without any additional heartbeat links, but it is necessary to add the third Witness node, which participates in the nodes count for the majority. The failure-handling process occurs when the node has detected the absence of connection with the partner.

The main requirement for keeping the node operational is an active connection with more than a half of the HA device’s nodes. Calculation of the available partners is based on their “votes”. In case of a two-node HA storage, all nodes will be disconnected if there is a problem on the node itself, or in communication between them. Therefore, the Node Majority failover strategy does not work in case of just two synchronous nodes. To resolve this issue, it is necessary to add the third Witness node, which participates in the nodes count for the majority, but neither contains data on it nor is involved in processing clients’ requests.

Note: With Node Majority failover strategy, failure of only one node can be tolerated. If two nodes fail, the third one will also become unavailable to clients’ requests.

Configure File Share (SMB) as Witness

That's another option. As per StarWind's help content.

Quote:

It is possible to configure SMB file share as Witness for StarWind HA device, created with Node Majority Failover Strategy. SMB Witness is a file, located on SMB share, which can be accessed by both nodes and help them to eliminate the split-brain issue in case of synchronization connection interruption between the nodes.

To set up the SMB file share as a Witness for 2-nodes HA device with Node Majority Failover Strategy, perform the following actions:

Create a folder, shared in the network, which will be used by StarWind VSAN as File Share Witness. Please note that the folder must be located out of StarWind nodes and a separated service account must be used by StarWind VSAN service to access the folder. It can be a local account as well as a domain account that has write permission to the folder. File share must be accessible from each StarWind node with credentials, mentioned above.
By default, samples are located under the path: C:\Program Files\StarWind Software\StarWind\StarWindX\Samples\powershell.

Locate the file named CreateHASmbWitness.ps1 and copy it to any other place for further editing.

Further reading here…

Final Words

Two-Node clusters are cost effective. Especially when you think that you can also avoid costs of switching (can direct-connect 2-nodes), those small architectures can save a lot of funds when deployed at larger scale with the need of having redundancy and keep an automated recovery when a temporary node failure occurs. Having everything on site, with hardware RAID protection, with StarWind VSAN heartbeat failover policy with automated synchronization of data between the two nodes automatically is a good way of limit admin's interactions.

Link: StarWind VSAN Product Page

More posts about StarWind on ESX Virtualization:

More posts from ESX Virtualization:

Stay tuned through RSS, and social media channels (Twitter, FB, YouTube)

Shares
5/5 - (1 vote)

About Vladan SEGET

This website is maintained by Vladan SEGET. Vladan is as an Independent consultant, professional blogger, vExpert x16, Veeam Vanguard x9, VCAP-DCA/DCD, ESX Virtualization site has started as a simple bookmarking site, but quickly found a large following of readers and subscribers.

Connect on: Facebook. Feel free to network via Twitter @vladan.

x