## 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)