## Overview
- SQL Server on Azure VMs uses Windows Server Failover Clustering (WSFC) for high availability.
- An FCI (Failover Cluster Instance) appears as a single SQL Server instance but provides failover capabilities.
- Supports disk witness, cloud witness, or file share witness for cluster quorum.
## Storage Options
- **Azure Shared Disks**: Supports SCSI Persistent Reservations, shared Premium SSD, and Ultra Disk storage.
- **Storage Spaces Direct**: Provides a software-based virtual SAN.
- **Premium File Shares**: SSD-backed with low latency.
- **Azure Elastic SAN**: Flexible, scalable, and cost-effective block storage.
## Connectivity
- Deploy SQL Server VMs to multiple subnets within the same virtual network for better failover performance.
## Limitations
- Limited extension support for SQL Server failover cluster instances on Azure VMs.
- MSDTC support varies based on Windows Server version and storage configuration.
## Resources
[Failover cluster instances - SQL Server on Azure VMs | Microsoft Learn](https://learn.microsoft.com/en-us/azure/azure-sql/virtual-machines/windows/failover-cluster-instance-overview?view=azuresql)