In the previous blog post, we discussed about what is vCLS. In this post, lets discuss some more features of it.
Cluster Service Health -
The agent VMs that form the cluster quorum state, are self correcting. This means that when the agent VMs are not available, vCLS will try to instantiate or power-on the VMs automatically.
There are 3 health states for the cluster services:
1. Healthy – The vCLS health is green when at least 1 agent VM is running in the cluster. To maintain agent VM availability, there’s a cluster quorum of 3 agent VMs deployed.
2. Degraded – This is a transient state when at least 1 of the agent VMs is not available but DRS has not skipped it’s logic due to the unavailability of agent VMs. The cluster could be in this state when either vCLS VMs are being re-deployed or getting powered-on after some impact to the running VMs.
3. Unhealthy – A vCLS unhealthy state happens when a next run of the DRS logic (workload placement or balancing operation) skips due to the vCLS control-plane not being available (at least 1 agent VM).
vSphere DRS for a cluster depends on the health of vSphere Cluster Services (vCLS). If vCLS health gets impacted due to unavailability of agent VMs in a cluster, then vSphere DRS will not be functional in the cluster until the time vCLS VMs are brought back up.
Retreat Mode -
During a normal operation, there is no way to disable the vCLS VM's and vCLS Services. If the agent VMs are missing or not running, the cluster shows a warning message.
However, should it be necessary, you can disable vCLS on a cluster using Retreat Mode, but keep in mind that cluster's services, such as DRS, will be impacted. Meaning that if you disable the provisioning of the VMs DRS will not work any longer, which also means that HA can’t leverage DRS for failover placement. Another important point to note here is that resource pools does not gets deleted when vCLS is disabled. Only DRS load balancing is impacted.
Login to the vSphere Client.
Navigate to the cluster on which vCLS must be disabled.
Copy the cluster domain ID from the URL of the browser. It should be similar to domain-c(number).
4. Navigate to the vCenter Server Configure tab.
5. Under Advanced Settings, click the Edit Settings button.
6. Add a new entry config.vcls.clusters.domain-c(number).enabled. Use the domain ID copied in step3
7. Set the Value to False.
8. Click Save.
As mentioned previously, vCLS monitoring service runs every 30 seconds. Within 1 minute, all the vCLS VMs in the cluster are cleaned up and the Cluster Services health will be set to Degraded. If the cluster has DRS enabled, it stops functioning and an additional warning is displayed in the Cluster Summary. DRS is not functional, even if it is enabled, until vCLS is reconfigured by removing it from Retreat Mode.
vSphere HA does not perform optimal placement during a host failure scenario. HA depends on DRS for placement recommendations. HA will still power on the VMs but these VMs might be powered on in a less optimal host.
To remove Retreat Mode from the cluster, change the value in step 7 to True.
Retreat Mode should be used with extra caution and should be used only for the purposes mentioned in the document.
Login to the vCLS VMs -
Ideally it is not recommended to login to the vCLS VM's as these VMs are managed by EAM. However if you want to login to the vCLS VM, maybe for some troubleshooting or for auditing, you can login using the below steps.
Use SSH to login to the vCenter Server Appliance.
Once logged in to the vCenter, run the following command, which then returns the password
/usr/lib/vmware-wcp/decrypt_clustervm_pw.py3. You can then SSH to one of the vCLS VM. Login with user root and the password retrieved from
step 2.
In the previous post we discussed about the datastore placement for the vCLS VM's. If you're using vSAN in your environment and have placed vCLS VM's on a vSAN Datastore you would need to be more cautious while working with vCLS VM's as some of the vSAN reconfigure workflows could be impacted if the vCLS VM's become unavailable for some reason. VMware has described the different scenarios of running vCLS VM's on vSAN in the KB.
With this I will conclude this series on VMware vSphere Cluster Services (vCLS).
I hope this was informative and thank you for reading!
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