VMware continuously enhances its vSphere platform to address scalability and reliability challenges in modern IT infrastructures. One key innovation is the vSphere Clustering Service (vCLS), introduced in vSphere 7 Update 1. Let’s explore how this service improves cluster operations and availability.
What is vCLS?
vCLS provides a distributed and decoupled control plane for clustering services, improving the reliability of features like the Distributed Resource Scheduler (DRS). Traditionally, clustering services depended on the vCenter Server’s availability. This dependency posed limitations on scalability and resilience. With vCLS, these challenges are mitigated, as it allows core clustering services to function even if the vCenter Server experiences downtime.
Architecture and Agent VMs
The vCLS architecture relies on lightweight agent VMs that are deployed automatically within each vSphere cluster. Key points include:
- Up to 3 agent VMs per cluster are created, forming a quorum to maintain service availability.
- In smaller clusters with fewer than 3 hosts, the number of agent VMs matches the number of ESXi hosts.
- These agent VMs are managed by vSphere and require no manual intervention from administrators.
Each vCLS agent VM runs a minimal configuration with a Photon OS, utilizing minimal resources:
- Memory: 128 MB (100 MB reserved)
- vCPU: 1 (100 MHz reserved)
- Disk: 2 GB (thin provisioned)
- No network adapter
Cluster Service Health
vCLS monitors and self-manages the availability of its agent VMs, with three health states:
- Healthy: At least one agent VM is active and functional.
- Degraded: One or more agent VMs are temporarily unavailable, though DRS operations continue.
- Unhealthy: DRS operations are interrupted due to a lack of available agent VMs.
If any agent VM becomes unavailable, vCLS automatically re-deploys or powers it on to maintain service integrity.
Operational Guidelines
Administrators do not need to maintain or interact with the agent VMs directly. The following practices help ensure smooth operations:
- Avoid deleting, renaming, or powering off agent VMs.
- When placing a host in maintenance mode, the agent VMs are automatically migrated to another host.
- Agent VMs are visible in the vSphere Client under the vCLS folder, but not in the Hosts and Clusters view.
Automation scripts should be configured to ignore vCLS agent VMs to prevent accidental disruptions. These VMs can be identified using specific properties such as ManagedByInfo.
Why vCLS Matters
vCLS significantly enhances the resilience and scalability of VMware clusters by enabling essential services to function independently of vCenter Server. This innovation reduces service downtime risks and optimizes operations in both on-premises and cloud environments.
By leveraging vCLS, organizations can achieve a higher level of automation, stability, and performance in their virtual infrastructure. VMware’s forward-thinking approach ensures that clusters remain robust and efficient, even under challenging conditions.
