How to fix Error 3303 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: Error 3303 — Remote Harden of Transaction Failed

Why This Error Happens

Error 3303 occurs when a transaction being hardened (written to the transaction log) on a remote secondary replica fails to complete within the expected time or encounters a write failure. This is typically caused by connectivity interruptions between Availability Group replicas, insufficient disk space or I/O bottlenecks on the secondary’s log drive, or corruption in the transaction log file itself. It can also surface when the secondary replica’s log drive has stale permissions preventing SQL Server’s service account from writing to the log path.

Step-by-Step Resolution Guide

Step 1: Verify Network Connectivity Between Replicas

  • Check the Always On Availability Group dashboard in SSMS for the affected database to confirm synchronization health.
  • Run Test-NetConnection -ComputerName <SecondaryReplicaName> -Port 5022 (or your configured endpoint port) from the primary replica in PowerShell.
  • Review Windows Server 2025 firewall rules under Windows Defender Firewall with Advanced Security to ensure the AG endpoint port is open inbound/outbound on both nodes.

Step 2: Inspect Disk Space and I/O Performance on the Secondary

  • Navigate to the log file directory (commonly D:\SQLData\Logs\ or your custom path) and confirm free disk space is above 15-20%.
  • Open Performance Monitor and add counters for Avg. Disk sec/Write and Avg. Disk Queue Length on the volume hosting the .ldf file.
  • If I/O latency exceeds 20ms consistently, consider moving the log file to a faster storage tier or SSD-backed volume.

Step 3: Check SQL Server Service Account Permissions

  • Open Services.msc, locate the SQL Server service, and confirm the account running it has Full Control on the log directory.
  • Use icacls "D:\SQLData\Logs\" /grant "NT SERVICE\MSSQLSERVER":(OI)(CI)F to reset permissions if needed.
  • Restart the SQL Server service after applying permission changes.

Step 4: Validate Transaction Log Integrity

  • Run DBCC CHECKDB ('YourDatabaseName') WITH NO_INFOMSGS, ALL_ERRORMSGS; to detect log or page-level corruption.
  • If corruption is found, restore the affected database from the last known good backup on the secondary replica.
  • Re-join the replica to the Availability Group using ALTER AVAILABILITY GROUP [AGName] JOIN; on the secondary instance.

Step 5: Review SQL Server Error Log and Windows Event Viewer

  • Open Event Viewer > Windows Logs > Application and filter for Event ID 3303 to correlate the exact timestamp with other AG-related errors (e.g., Event ID 35201, 41414).
  • Cross-reference the SQL Server Error Log (ERRORLOG file in MSSQL\Log directory) for additional context around the failed LSN.

Alternative or Verification Steps

  • After applying fixes, manually force a log flush by running a small transaction on the primary and confirm it hardens successfully on the secondary via the AG dashboard (status should show “Synchronized”).
  • If the issue persists, consider temporarily removing the problematic secondary from the Availability Group, running a fresh full backup/restore, and re-adding it to rule out underlying log corruption.