How to fix Error 3871 on Windows Server 2025 / Windows 11

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Why This Error Happens

Error 3871 is logged when SQL Server 2025 initializes tempdb using the Memory-Optimized TempDB Metadata feature, which moves system metadata tables (like DMVs tracking session and transaction info) into lock-free, latch-free memory-optimized structures. This typically occurs because trace flag 49993 is enabled globally, or because the MEMORY_OPTIMIZED = ON configuration was explicitly set via ALTER SERVER CONFIGURATION. While this message is often informational and indicates improved performance under high concurrency, it can surface as an unexpected error if the feature was enabled unintentionally, or if downstream monitoring scripts/third-party tools aren’t compatible with the altered system catalog behavior.

Step-by-Step Resolution Guide

Step 1: Confirm the Current TempDB Configuration State

  • Open SQL Server Management Studio (SSMS) and connect to the instance.
  • Run the following query to check if the feature is active:
  SELECT SERVERPROPERTY('IsTempdbMetadataMemoryOptimized') AS IsMemoryOptimized;
  • A return value of 1 confirms the feature is enabled and is the direct cause of Event 3871.

Step 2: Review Trace Flags and Startup Parameters

  • Check SQL Server Configuration Manager (via Windows Server 2025 or Windows 11) under SQL Server Services > Properties > Startup Parameters.
  • Look for -T49993 listed as an active trace flag.
  • If present and unintended, remove the trace flag entry from the startup parameters list.

Step 3: Disable Memory-Optimized Metadata (If Unwanted)

  • If the feature was enabled deliberately via T-SQL rather than a trace flag, disable it with:
  ALTER SERVER CONFIGURATION SET MEMORY_OPTIMIZED TEMPDB_METADATA = OFF;
  • Note: This change requires a full restart of the SQL Server service to take effect, as tempdb is rebuilt at every startup.
  • Restart via PowerShell: Restart-Service -Name 'MSSQLSERVER'

Step 4: Validate File Paths and Permissions for TempDB

  • Navigate to the default tempdb path (e.g., C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\DATA).
  • Ensure the SQL Server Service Account has Full Control (Read/Write/Modify) permissions on this directory to prevent corruption during the memory-optimized filegroup creation process.
  • If permissions were recently changed by Group Policy on Windows Server 2025, re-apply the local security policy for the SQL service account.

Alternative or Verification Steps

  • Verify the Fix: After restarting the service, re-run SELECT SERVERPROPERTY('IsTempdbMetadataMemoryOptimized');. If you disabled the feature in Step 3, this should now return 0, and Event ID 3871 should no longer appear in the SQL Server Error Log upon restart.
  • Alternative Solution: If the error persists despite disabling the server configuration, check for a lingering model database corruption issue, as tempdb inherits its initial structure from model. Running DBCC CHECKDB on the model database can rule out underlying corruption forcing the fallback behavior.