In a previous blog, we talked about vSAN Stretched Clusters. In this blog, let's discuss more about some best practices and design considerations when working with stretched clusters.
Consider the following recommendations while working with a vSAN stretched cluster.
- For stretched clusters, configure the Primary level of failures to tolerate to 1.
- Stretched clusters require on-disk format 2.0 or later. If necessary, upgrade the on-disk format before configuring a stretched cluster.
- Symmetric multiprocessing fault tolerance(SMP-FT) is not supported by vSAN stretched clusters.
- Using esxcli to add or remove hosts is not supported for stretched clusters.
- When a host is disconnected or not responding, you cannot add or remove the witness host. This limitation ensures that Virtual SAN collects enough information from all hosts before initiating reconfiguration operations.
- Configure HA settings for the stretched cluster.
HA must be enabled on the cluster.
HA rule settings should respect VM-Host affinity rules during failover.
Disable HA datastore heartbeats.
- Configure DRS settings for the stretched cluster.
DRS must be enabled on the cluster. If you place DRS in partially automated mode, you can control which VMs to migrate to each site.
Create two host groups, one for the preferred site and one for the secondary site.
Create two VM groups, one to hold the VMs on the preferred site and one to hold the VMs on the secondary site.
Create two VM-Host affinity rules that map VMs-to-host groups, and specify which VMs and hosts reside in the preferred site and which VMs and hosts reside in the secondary site.
Configure VM-Host affinity rules to perform the initial placement of VMs in the cluster.
Network Design considerations -
A vSAN stretched cluster must adhere to some basic networking requirements -
- Management network requires connectivity across all three sites, using a Layer 2 stretched network or a Layer 3 network.
- The vSAN network requires connectivity across all three sites. It must have independent routing and connectivity between the data sites and the witness host. vSAN supports both Layer 2 and Layer 3 between the two data sites, and Layer 3 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.
Best Practices for Working with Stretched Clusters -
For optimal performance while using vSAN extended clusters, abide by following suggestions.
- If a host is permanently unavailable, remove the host from the cluster before you perform any reconfiguration tasks.
- If one of the sites (fault domains) in a stretched cluster is inaccessible, new VMs can still be provisioned in the subcluster containing the other two sites. These new VMs are implicitly force provisioned and are non-compliant until the partitioned site rejoins the cluster. This implicit force provisioning is performed only when two of the three sites are available. A site here refers to either a data site or the witness host.
- If an entire site goes offline due to a power outage or loss of network connection, restart the site immediately, without much delay. Instead of restarting vSAN hosts one by one, bring all hosts online approximately at the same time, ideally within a span of 10 minutes. By following this process, you avoid resynchronizing a large amount of data across the sites.
- If you want to clone a VM witness host to support multiple stretched clusters, do not configure the VM as a witness host before cloning it. First deploy the VM from OVF, then clone the VM, and configure each clone as a witness host for a different cluster. Or you can deploy as many VMs as you need from the OVF, and configure each one as a witness host for a different cluster.
With this I'll conclude this post here.
Thank you for reading!
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