In a recent webinar, Randy Knight walked through the backup enhancements in SQL Server 2025—and one update in particular changes everything for Availability Groups.
True Full and Differential Backups on Secondary Replicas
For years, database professionals faced a frustrating limitation. With Availability Groups, you could offload some workloads to secondary replicas—but not your real backup strategy.Yes, you could run copy-only backups on a secondary. But true full backups? Differential backups that maintain the LSN chain? Those had to run on the primary replica, until now. SQL Server 2025 introduces the ability to run true full backups and true differential backups on secondary replicas.
That means:
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You can move your entire backup workload off the primary.
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Your backup plan behaves exactly the same as if it were running on the primary.
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You potentially reduce your RPO by running backups more frequently.
This isn’t just a convenience feature—it’s a performance and resilience upgrade.
ZSTD Backup Compression
Another enhancement is the introduction of ZSTD backup compression, an algorithm originally developed at Facebook. Until now, SQL Server used MSXpress as the default compression method. ZSTD brings:
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Slightly better compression ratios
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Faster compression in many cases
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Less CPU overhead during decompression
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Potentially faster restores
In Randy’s testing—and in benchmarking done by Aaron Bertrand on an 8TB Stack Overflow database—backup sizes weren’t dramatically smaller. A 50GB database might shrink from 15GB to 14.5GB. That doesn’t sound revolutionary, but at scale it really adds up!
If you’re backing up multi-terabyte databases with long retention periods, even small savings compound quickly. Smaller backups also mean improved storage efficiency and potential restore performance gains. It’s not flashy—but it’s practical, which matters.
Immutable Backups in Azure Blob Storage
Backing up directly to Azure Blob Storage isn’t new. But SQL Server 2025 now supports backups to immutable Azure Blob storage accounts.This enhancement strengthens ransomware protection. Immutable storage ensures that once a backup is written, it cannot be modified. The only permitted action is deletion under defined rules. While not truly air-gapped, this approach creates a powerful safeguard against backup tampering. In a world where ransomware targets backups first, this feature significantly improves recovery confidence.
Why These Enhancements Matter
SQL Server 2025 doesn’t reinvent backup strategy—it strengthens it.
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Offload backup workloads to secondary replicas
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Improve compression efficiency at scale
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Harden backup security against ransomware
Individually, these updates are incremental. Together, they represent a meaningful evolution in database resilience and performance. For organizations running mission-critical systems, that’s an especially big deal.
