Information2 use cookies to help you have a superior and more admissible browsing experience on our website. Privacy Policy
Loading...
By:
SQL Server Always On Availability Groups (AG) provide high availability and disaster recovery by continuously synchronizing databases between primary and secondary replicas. However, administrators may encounter an issue where the secondary database status changes to NOT SYNCHRONIZING, causing data replication delays and affecting failover readiness.
The SQL Server Always On database not synchronizing issue can happen due to multiple reasons, including suspended data movement, transaction log transmission failures, HADR endpoint problems, network interruptions, or secondary database recovery failures.
This guide explains what the Always On database not synchronizing status means, how to identify the root cause, and how to fix SQL Server Availability Group synchronization problems step by step.
When an Always On Availability Group database is not synchronizing, follow these troubleshooting steps:
In most cases, running diagnostic queries against SQL Server Always On DMVs can quickly identify why the secondary database stopped synchronizing.
In SQL Server Always On Availability Groups, database synchronization status represents whether transaction log changes from the primary replica are successfully transferred and applied to secondary replicas.
The synchronization states include:
| Synchronization State | Description |
|---|---|
| SYNCHRONIZED | Primary and secondary databases are fully synchronized |
| SYNCHRONIZING | Secondary replica is receiving and applying transaction logs |
| NOT SYNCHRONIZING | Secondary replica cannot synchronize transaction log changes |
When an Always On Availability Group database is not synchronizing, the secondary replica is no longer maintaining an up-to-date copy of the primary database.
This means:
The issue is especially critical in production environments where SQL Server Always On is used for business continuity.
There are several common reasons why an Always On secondary database is not synchronizing.
One of the most common causes is suspended data movement.
When data movement is suspended, SQL Server stops sending transaction logs from the primary replica to the secondary replica.
This may happen because of:
You can check whether synchronization is suspended by running:
SELECT
DB_NAME(database_id) AS DatabaseName,
is_suspended,
suspend_reason_desc
FROM sys.dm_hadr_database_replica_states;
If the database is suspended, resume synchronization:
ALTER DATABASE [DatabaseName]
SET HADR RESUME;
Always On Availability Groups rely on database mirroring endpoints (HADR endpoints) to transfer transaction logs between replicas.
If the endpoint is stopped or inaccessible, the secondary replica cannot receive updates.
Common causes include:
Check endpoint status:
SELECT
state_desc,
role_desc
FROM sys.database_mirroring_endpoints;
The endpoint should normally show:
STARTED
If needed, restart the endpoint:
ALTER ENDPOINT [Hadr_endpoint]
STATE = STARTED;
SQL Server Always On synchronization depends on continuous transaction log movement.
If the primary replica generates logs faster than the secondary can process them, synchronization may stop.
Common causes include:
Check log send and redo queues:
SELECT
DB_NAME(database_id) AS DatabaseName,
log_send_queue_size,
redo_queue_size
FROM sys.dm_hadr_database_replica_states;
Important metrics:
Large queue values indicate synchronization delays.
Another common scenario is:
Synchronization State:
NOT SYNCHRONIZING
Database State:
RECOVERY_PENDING
This usually occurs after:
SQL Server cannot bring the secondary database online, preventing synchronization.
Check database state:
SELECT
name,
state_desc
FROM sys.databases;
If the database remains in recovery pending state, review SQL Server error logs and investigate:
A corrupted secondary database may enter:
SUSPECT
state.
When this happens, SQL Server cannot complete recovery, and Always On synchronization stops.
Possible causes include:
Check database consistency:
DBCC CHECKDB ([DatabaseName]);
If corruption is confirmed, restoring or reseeding the secondary database is usually safer than forcing repairs.
Before applying fixes, administrators should first identify the current synchronization condition.
Run the following query:
SELECT
DB_NAME(database_id) AS DatabaseName,
synchronization_state_desc,
synchronization_health_desc,
database_state_desc
FROM sys.dm_hadr_database_replica_states;
The result helps determine whether the issue is related to:
You can also check Availability Group replica health:
SELECT
replica_server_name,
connected_state_desc,
synchronization_health_desc
FROM sys.dm_hadr_availability_replica_states;
There are some methods you can try to fix SQL Server always on database not synchronizing.
If synchronization was suspended, resume it manually.
Command:
ALTER DATABASE [DatabaseName]
SET HADR RESUME;
After running the command, monitor:
If replicas cannot communicate, restart the endpoint.
Check status:
SELECT *
FROM sys.database_mirroring_endpoints;
Start endpoint:
ALTER ENDPOINT [Hadr_endpoint]
STATE = STARTED;
Also verify:
If the secondary replica cannot keep up with transaction log changes:
Check:
Possible actions:
If the secondary database remains unhealthy, removing and joining it again may restore synchronization.
Remove database from AG:
ALTER DATABASE [DatabaseName]
SET HADR OFF;
Restore the database on the secondary replica and join it back:
ALTER DATABASE [DatabaseName]
SET HADR AVAILABILITY GROUP = [AG_Name];
If the secondary database is damaged or too far behind, performing a new seed operation may be required.
Common reseeding methods include:
After reseeding, verify:
synchronization_state_desc = SYNCHRONIZED
Preventing synchronization failures requires continuous monitoring and proper SQL Server HA management.
Recommended practices:
Regularly check:
Slow disks can delay transaction log replay and cause synchronization lag.
Unexpected log growth can create synchronization bottlenecks.
Automated monitoring helps detect:
For organizations running mission-critical SQL Server workloads, maintaining continuous availability requires more than database replication monitoring.
Information2 Software provides solutions designed for enterprise data availability and disaster recovery.
i2Availability helps businesses protect critical applications through real-time data replication, automatic failover, and continuous availability mechanisms.
With i2Availability, organizations can:
1. Why is my SQL Server Always On database stuck in NOT SYNCHRONIZING?
The most common causes are suspended data movement, HADR endpoint failures, transaction log transmission issues, network problems, or secondary database recovery failures.
2. How do I check Always On synchronization status?
You can check synchronization status using:
sys.dm_hadr_database_replica_states
This DMV shows synchronization state, health, and queue information.
3. How do I resume SQL Server Always On synchronization?
Run:
ALTER DATABASE [DatabaseName]
SET HADR RESUME;
This resumes data movement between replicas.
4. Can network issues cause Always On database not synchronizing?
Yes. Always On Availability Groups require continuous communication between replicas. Network interruptions can prevent transaction logs from reaching secondary replicas.
5. Does rebuilding the secondary database fix synchronization problems?
If the secondary database is corrupted or significantly behind, removing and reseeding the secondary replica can restore synchronization.
