vSAN availability technologies are designed to ensure data resilience and minimize downtime in the event of failures. These technologies work in tandem with VMware vSphere HA and the storage policy-based management (SPBM) framework to maintain the integrity of data and the availability of virtual machines. Understanding how vSAN handles failures at various levels is critical to maintaining a highly available environment.
vSAN achieves data availability by distributing data components across multiple hosts and fault domains based on policies. The most commonly used policy is failures to tolerate. This defines the number of host, disk, or fault domain failures a virtual machine object can withstand without data loss. For example, a RAID-1 mirror with one failure to tolerate (FTT=1) stores two copies of data on separate hosts. If one host fails, the other retains availability.
Beyond mirroring, vSAN also supports erasure coding with RAID-5 and RAID-6 configurations. RAID-5 supports one failure while RAID-6 tolerates two. These configurations offer space efficiency but require more hosts to satisfy policy requirements. Erasure coding is ideal for capacity-focused workloads that can tolerate slightly higher write latencies compared to mirroring.
One of the key availability mechanisms is vSAN’s ability to detect and respond to failures. When a component becomes inaccessible, vSAN determines whether the failure is transient or permanent. It uses a time-based mechanism to wait before rebuilding data, preventing unnecessary resyncs due to brief outages. The default delay is 60 minutes, after which a repair operation may commence if the component remains unavailable.
In addition to failure handling, vSAN includes automatic rebalancing features. When disk space across the cluster becomes uneven or after repairs, vSAN may redistribute components to optimize performance and avoid hotspots. This helps ensure balanced use of storage resources and sustained performance.
Witness components play an important role in maintaining quorum during failures. These are small metadata objects that participate in cluster decisions. For example, in a 2-node configuration, a witness resides on a separate host or appliance to act as a tie-breaker. This ensures that split-brain scenarios are avoided, and availability decisions are made accurately.
vSAN also incorporates features to protect against storage device failures. It can mark devices as degraded or failed and migrate components as needed. When a disk exhibits signs of wear or IO errors, vSAN initiates evacuation processes and alerts administrators via health checks and vCenter alarms.
Stretched cluster configurations allow for availability across two sites with a third witness site. Each site has its own fault domain, and vSAN ensures synchronous writes between them. In the event of a site failure, vSAN maintains availability by failing over to the other site. The witness in a third site helps determine which site should remain active, ensuring consistency and preventing data corruption.
Maintenance mode operations are also availability-aware. vSAN supports three modes: ensure accessibility, evacuate all data, and no data migration. The chosen mode affects whether VMs remain accessible during host maintenance and how data is protected. For example, ensure accessibility keeps data available but may reduce resilience until the host returns. Evacuate all data fully preserves availability by migrating components off the host in advance.
Proactive monitoring and alerting are critical to vSAN availability. Skyline Health checks for cluster, network, and hardware issues, while vCenter provides event logs and alerts for deeper investigation. Administrators can define alarms for capacity thresholds, degraded hardware, and other risk indicators to prevent outages.
In summary, vSAN offers a comprehensive set of availability technologies that span hardware failure detection, policy-driven resilience, automated repair and rebalancing, and integration with vSphere HA. These mechanisms ensure that virtualized workloads remain protected, resilient, and highly available, even in the face of component or site failures.
