Data Redundancy in Azure Files

Azure Files stores multiple copies of data to meet availability and durability targets and protect against hardware failures, network/power outages, and regional disasters. When choosing a redundancy option, balance cost vs. availability and consider how data is replicated in the primary region and whether geo-replication to a distant secondary region is required.

Key points

Redundancy is set at the storage account level; all file shares in the same account share the same redundancy. Use separate storage accounts for different redundancy requirements.
Primary-region replication options:
- Locally redundant storage (LRS)
  - Replicates data three times within one or more availability zones (or within a single data center).
  - Durability: at least 99.999999999% (11 9's) per year.
  - Lowest cost; protects against rack/drive failures but not regional disasters.
  - Write operations are synchronous (returns after written to all three replicas).
  - Supported in all regions for HDD file shares; check LRS support for SSD file shares here: https://docs.azure.cn/en-us/storage/files/redundancy-premium-file-shares#lrs-support-for-ssd-azure-file-shares
- Zone-redundant storage (ZRS)
  - Replicates synchronously across three availability zones in the primary region.
  - Durability: at least 99.9999999999% (12 9's) per year.
  - Provides read/write availability if a single zone becomes unavailable; write operations are synchronous across zones.
  - Recommended for high-availability scenarios and for use with Azure File Sync.
  - Check ZRS SSD support: https://docs.azure.cn/en-us/storage/files/redundancy-premium-file-shares#zrs-support-for-ssd-azure-file-shares
Secondary-region (geo) replication options (only for standard HDD SMB file shares):
- Geo-redundant storage (GRS)
  - Primary region: LRS replication (three synchronous copies).
  - Asynchronously replicates to a secondary region; secondary region also stores three LRS copies.
  - Durability: at least 99.99999999999999% (16 9's).
- Geo-zone-redundant storage (GZRS)
  - Primary region: ZRS across availability zones.
  - Asynchronously replicated to a secondary region (secondary uses LRS).
  - Combines zone-level HA with protection from regional disasters; recommended for maximum availability/durability.
- Note: SSD file shares do not support GRS/GZRS. Alternatives include Azure File Sync to another region (disable cloud tiering and access via server endpoint) or scripts/tools such as AzCopy (v10.4+ to preserve ACLs/timestamps).
Snapshot and sync behavior for geo options
- System snapshot in primary region is created every 15 minutes and replicated to secondary to keep file shares consistent for failover.
- Last Sync Time (LST) indicates last successful write to secondary (based on system snapshot). LST:
  - Is based on the service in the account that is furthest behind.
  - Doesn't update if no changes occurred.
  - May time out when >100 file shares — recommended <100 file shares per account.
Failover considerations
- Secondary region is not accessible unless you perform an account failover.
- Failover updates DNS so secondary becomes primary; data is inaccessible during failover.
- Constraints and impacts:
  - Failover blocked if no system snapshot exists in the secondary region.
  - Failover blocked if account contains >100,000 file shares (support request required).
  - File handles and leases are not retained; clients must unmount/remount.
  - File share quotas may change to values from latest snapshot.
  - In-progress copy operations are aborted and must be retried after failover.
- Azure Files does not support RA-GRS or RA-GZRS; RA variants configured will be treated/billed as GRS/GZRS.
- Warning: directory queries and performance (especially for directories with many files or many snapshots) can experience higher latency after failover and during resynchronization.
Durability/availability summary (high-level)
- LRS: ~11 9's durability; ZRS: ~12 9's; GRS/GZRS: ~16 9's.
- Read/write availability: generally at least 99.9% (99% for Cool tier).
- ZRS and GZRS offer protection against zone outages; GRS/GZRS offer protection against region-wide outages (failover required for writes).
Region and billing model support
- Region support varies by storage account kind, billing model, and media tier. Use the Portal, PowerShell, or Azure CLI to query supported SKUs and regions (examples provided in the original article).