In SQL Server environments where transactional replication runs alongside Always On Availability Groups (AGs), DBAs sometimes face a frustrating scenario: replication stalls when a secondary replica or subscriber is offline for maintenance, patching, or unexpected downtime.
By default, SQL Server’s Log Reader Agent is cautious. It only marks transactions as ready for replication once they are hardened on both the primary and all replicas. This ensures consistency across the AG, but it can also cause replication to stall if an asynchronous replica or subscriber is unavailable for an extended period of time.
The Challenge: Offline Subscribers or Replicas
Consider this scenario:
- You have a transactional replication publisher that is also part of an Availability Group.
- One of your asynchronous replicas or subscribers is intentionally offline (for patching, DR testing, or migrations).
- The Log Reader Agent refuses to advance past the last acknowledged transaction, effectively blocking replication until the offline node comes back online.
For high-throughput systems, this can mean hours or days of replication backlog, frustrating downstream consumers who rely on timely data.
Enter Trace Flag 1448
Enabling Trace Flag 1448 changes this behavior:
- The Log Reader Agent continues processing transactions for replication even if asynchronous replias or subscribers are offline.
- Replication latency is no longer tied to the availability of secondaries.
- This ensures that subscribers remain current while offline nodes are unavailable.
From Microsoft’s documentation and field experience:
- Synchronous replicas are still honored. The Log Reader will not move past uncommitted transactions on synchronous secondaries.
- Asynchronous replicas are ignored for replication purposes when the trace flag is enabled.
How to Enable
You can enable Trace Flag 1448 dynamically or persistently:
-- Enable globally without restart DBCC TRACEON(1448, -1); -- Disable globally DBCC TRACEOFF(1448, -1);
Or add
-T1448 to SQL Server startup parameters for persistence across service restarts.
Risks and Considerations
While Trace Flag 1448 solves replication latency issues, it comes with trade-offs:
- Failover Risk: If a failover occurs while an async replica is behind, the new primary may not contain all transactions already replicated to subscribers. This can cause replication inconsistency and may require reinitialization.
- Operational Awareness: DBAs must monitor replication health closely when using this trace flag, especially during planned or unplanned failovers.
- Use Case Specific: Best suited for environments where replication freshness is critical and the business can tolerate the risk of reinitialization after failover.
Best Practices
- Use Trace Flag 1448 only when the subscribers or async replicas are expected to be offline for extended periods.
- Combine with robust monitoring for replication latency and AG health.
- Document the operational risk so that application owners understand the trade-off.
- Test failover scenarios in a non-production environment to validate recovery procedures.
Conclusion
Trace Flag 1448 is a powerful but situational tool. It allows replication to continue flowing even when subscribers or asynchronous replicas are offline, preventing massive replication backlogs. However, it shifts the burden to DBAs to manage the risk of failover inconsistencies.
For organizations where replication latency is business-critical, enabling Trace Flag 1448 can be the difference between smooth operations and hours of catch-up chaos.
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