vSAN Stretched Cluster was first introduced with vSAN 6.1. vSAN Stretched cluster allows you to configure vSAN Cluster between 2 datacenters where the distance between data centers is limited, such as metropolitan or campus environments.
A vSAN Stretched Cluster is a specific configuration implemented in environments where disaster/downtime avoidance is a key requirement. You can use stretched clusters to manage planned maintenance and avoid disaster scenarios, because maintenance or loss of one site does not affect the overall operation of the cluster. In a stretched cluster configuration, both data sites are active sites. If either site fails, vSAN uses the storage on the other site.
Below is the sample Architecture of vSAN Stretched Cluster:
vSAN Stretched Clusters with a Witness Host refers to a deployment where a user sets up a vSAN cluster with 2 active/active sites with an identical number of ESXi hosts distributed evenly between the two sites. The sites are connected via a high bandwidth/low latency link.
The third site hosting the vSAN Witness Host is connected to both of the active/active data-sites. This connectivity can be via low bandwidth/high latency links. Each stretched cluster consists of two data sites and one witness host. The witness host resides at a third site and contains the witness components of virtual machine objects. It contains only metadata, and does not participate in storage operations. The witness host serves as a tiebreaker when a decision must be made regarding availability of datastore components when the network connection between the two sites is lost.
A virtual machine deployed on a vSAN Stretched Cluster will have one copy of its data on site A, a second copy of its data on site B and any witness components placed on the witness host in site C.
A vSAN stretched cluster must meet certain basic networking requirements:
Management network requires connectivity across all three sites, using a Layer 2 stretched network or a Layer 3 network.
vSAN network requires connectivity across all three sites. It must have independent routing and connectivity between the data sites and the witness host. Use a Layer 2 stretched network between the two data sites and a Layer 3 network between the data sites and the witness host. vSAN network requires connectivity across all three sites. It must have independent routing and connectivity between the data sites and the witness host. Use a Layer 2 stretched network between the two data sites and a Layer 3 network between the data sites and the witness host.
VM network requires connectivity between the data sites, but not the witness host. Use a Layer 2 stretched network or Layer 3 network between the data sites. In the event of a failure, the VMs do not require a new IP address to work on the remote site.
vMotion network requires connectivity between the data sites, but not the witness host. Use a Layer 2 stretched or a Layer 3 network between data sites.
Before going for Stretched Cluster configuration make sure you have Deployed & configured vSAN Witness Host & configured Fault Domain. If you have not configured yet then you can check below links to configure same:
So let’s see How to Configure vSAN Stretched Cluster Step by Step:
Login to vSphere web client.
Select your vSAN Cluster. Go to Configure Tab.
Select Fault Domains under vSAN Section.
Here you will see Stretched Cluster is currently disabled.
Click Configure to configure stretched cluster.
You will have to configure Preferred & secondary fault domain here.
Based on your environment select Fault Domain and move to Secondary fault domain using arrows.
Click Next once you configure your Preferred & Secondary Fault domain.
Select the Witness host which you have deployed & configured.
Make sure it fulfills all the requirements listed.
Here you need to claim disks for witness host.
Claim disk for cache tier & capacity tier.
This disks won’t be used to store VM disks , it will only be used to store witness components.
Review the complete configuration & click Finish to start the Stretched Cluster configuration process.
It may take time based on your environment , you can view the status of the process & different tasks in the Recent Tasks.
Once it is configured successfully you can see that Stretched Cluster is enabled & Preferred & Secondary Fault Domain is configured.
So using this way you can easily configure vSAN Stretched Cluster between two datacenters. Stretched Cluster is very useful when your one of datacenter is not available so at that time you will have all your virtual machines from secondary datacenter available.
We recommend you to check for complete system & network requirements for vSAN Stretched Cluster with VMware to avoid any issues later.
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Information source: VMware Documentation.