VSAN Stretched Clusters – Click Click done!

Creating Stretched Clusters could not be easier with VSAN. In fact the process of configuring different types of VSAN clusters, with features like deduplication and compression enabled is now a snap in the new Configuration Wizard.

In this article  will spend a few minutes walking through the wizard to get my Stretched Cluster setup. But first here is a quick description of my environment. Pretty standard, 2 DC’s in the metro area about 30km apart and a head office where I will run my witness vm.

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So let’s get started. First you need to download and deploy the witness appliance. It’s a basic appliance configuration and will ask you what size appliance to create based on the number of VM’s to be managed. Grab it from the vSphere download link on the website.

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As it stands I have 4 hosts in the cluster but am yet to configure VSAN. Commencing the configuration wizard we are asked if we want to create a standard, 2 node or Stretched Cluster. In VSAN 6.2, as mentioned before, we streamlined the configuration of all of these deployments with one wizard.

Here I’m simply going to select Automatic claiming of my disks and select Stretched Cluster.

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We now have validation of the VSAN networks for each host in the cluster and I can see clearly my VSAN VMkernel network has been correctly configured, as per any normal VSAN host.

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We now have validation of the VSAN networks for each host in the cluster and I can see clearly my VSAN VMkernel network has been correctly configured, as per any normal VSAN host.

Firstly I have renamed both of my Fault Domains to reference the location of the Datacenters. Initially I need to place the hosts into the correct Fault Domain and I can do this with a single click.

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So now my Fault Domain mirror what my physical deployment represents. I have two Dell hosts in the City Datacenter and two in the Clayton Datacenter.

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Of course with VSAN Stretched Clusters I require access to the Witness appliance which I downloaded an deployed earlier. This  appliance actually runs back at my main site. Here in this step I simply need to select the Witness for the cluster. You can see that this Witness is essentially a virtual ESXi host however we have made it clear to differentiate by using the blue icon.

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Depending on my choices I may need to ensure I claim the storage on this Witness appliance. It is important to do this as VSAN will today always expect to find the a similar configuration across data nodes and witness nodes. Hence we have created the appliance with a Cache disks and a Capacity disk.

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Verify the settings and complete.

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Now if we navigate over to the Stretched Cluster Management we can verify that it is correctly enabled. We also can see that the preferred Fault Domain is the Clayton Datacenter. This means in the event of a split-brain scenario the Clayton Datacenter and the Witness will form a quorum and the City Datacenter will be isolated. In this case HA will power on VM’s in the Clayton Datacenter. The Preferred Site is also denoted by the yellow star on the Clayton node.

Today a Stretched Cluster can maintain only one failure however other mechanisms such as SRM and VSAN Replication can be used to provide further protection against failures if required.

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Monitoring the health of a Virtual SAN cluster is paramount and a Stretched Cluster is no exception. In VSAN we now have some additional information in the Health Checks which help to monitor a Stretched Cluster effectively. Below is a grab of the checks we now have to ensure you stay happy and healthy.

I have introduced some artificial latency too demonstrate what happens here. As you will note the requirement for RTT latency between the two datacenter is documented as up to 5ms.

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If I increase the latency I would see an error such as the one below and in addition in VSAN 6.2 now we also have Health Alarms on the Summary pages

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Alarms can be programmed to send Email or SNMP type alerts so other people or systems can get notification when things aren’t working correctly.

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Other Considerations for Stretched Clusters might be different networking topologies, Host Groups and Rules, HA settings. All are covered in the Stretched Cluster guide below.


There is a wealth of information available for Stretched Clusters today

VSAN 6.2 Stretched Cluster Guide https://www.vmware.com/files/pdf/products/vsan/VMware-Virtual-SAN-6.2-Stretched-Cluster-Guide.pdf

VSAN Stretched Cluster Bandwidth and Sizing Guide http://www.vmware.com/files/pdf/products/vsan/vmware-virtual-san-6.1-stretched-cluster-bandwidth-sizing.pdf

VSAN Stretched Cluster Performance and Best Practices http://www.vmware.com/files/pdf/techpaper/vmware-virtual-san-stretched-cluster.pdf

VSAN Stretched Clusters and SRM https://www.vmware.com/files/pdf/techpaper/Stretched_Clusters_and_VMware_vCenter_Site_Recovery_Manage_USLTR_Regalix.pdf

Designing a VSAN Stretched Cluster http://www.yellow-bricks.com/2015/09/23/designing-a-virtual-san-stretched-cluster

VSAN Stretched Cluster demo http://www.yellow-bricks.com/2015/09/10/virtual-san-stretched-clustering-demo

VSAN Stretched Cluster supported network topologies http://cormachogan.com/2015/09/10/supported-network-topologies-for-vsan-stretched-cluster


Hopefully you can now see how simple and intuitive it is to create VSAN clusters now with 6.2.