MS SQL backup immutability

You may need to be aware below in terms of MS SQL immutability backup:

•    AG complexity: Always On AGs involve multiple replicas, and backup coordination across nodes (especially with backup preferences) adds layers of orchestration.
•    VM-level backups: Hypervisor-based backups (e.g., via VSS snapshots) often lack application-level immutability and can cause cluster instability if not configured precisely.
•    SQL-native backups: While SQL Server supports backup encryption and compression, it doesn't natively enforce immutability post-write.

Viable Approaches to Immutability
1. Azure Backup with Recovery Services Vault
•    Supports SQL Server AGs if all nodes are in the same region and subscription.
•    Offers soft delete and immutable vaults to prevent tampering or deletion of backups.
•    Backup jobs run on the primary node for full/diff backups; secondary replicas may be used for copy-only/log backups.
2. Immutable Storage (Blog Storage)
•    Store backups in immutable blob containers (Azure).
•    Use SQL backup to disk, then move to immutable storage via automation.
•    Pros: Strong immutability guarantees.
•    Cons: Adds complexity and delay; not real-time immutability.
3. Third-party Backup Solutions
•    Vendors like Veeam that offer AG-aware backups with optional immutability layers.
•    Some support image-level backups with retention locks.
•    Caveats: Licensing, configuration overhead, and varying support for AG nuances.

Best Practices
•    Use copy-only backups to avoid interfering with transaction log chains.
•    Automate post-backup transfer to immutable storage.
•    Enable backup encryption and restrict access to backup folders.
•    Test restore scenarios regularly to validate integrity and retention.