How to fix Error 8991 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 8991 – Invalid Address Range

Why This Error Happens

Error 8991 occurs when SQL Server’s memory manager detects that a pointer address range passed during a memory allocation or deallocation operation does not correspond to a valid, previously allocated region. This is typically caused by database corruption affecting in-memory OLTP (Hekaton) structures, a mismatch between SQL Server’s memory-mapped file pointers and actual committed memory, or corrupted buffer pool pages. It can also surface after improper shutdowns, faulty RAM, or incompatible third-party memory-scanning tools (such as antivirus or endpoint protection software) interfering with the sqlservr.exe process address space.

Step-by-Step Resolution Guide

Step 1: Check for Hardware-Level Memory Corruption

  • Run Windows Memory Diagnostic Tool by executing mdsched.exe from the Run dialog on Windows Server 2025 / Windows 11.
  • Review results in Event Viewer under Windows Logs > System after reboot, filtering for source “MemoryDiagnostics-Results”.
  • If errors are found, contact your hardware vendor to replace faulty DIMMs before proceeding further.

Step 2: Run DBCC CHECKDB to Identify Corruption

  • Open SQL Server Management Studio (SSMS) and connect to the affected instance.
  • Execute the following command against the database reporting the error:
DBCC CHECKDB ('YourDatabaseName') WITH NO_INFOMSGS, ALL_ERRORMSGS;
  • If corruption is found in In-Memory OLTP filegroups, note the affected table/checkpoint file group for targeted repair.

Step 3: Exclude SQL Server Processes from Antivirus/EDR Scanning

  • Navigate to Windows Security > Virus & threat protection > Manage settings > Exclusions.
  • Add exclusions for:
  • C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Binn\sqlservr.exe
  • The data and log file directories (e.g., D:\SQLData\ and L:\SQLLogs\)
  • Restart the SQL Server service via services.msc to apply the exclusion cleanly.

Step 4: Review and Adjust Max Server Memory Configuration

  • Open SSMS, right-click the server instance, and select Properties > Memory.
  • Ensure “Maximum server memory” is set below total physical RAM (leave at least 4GB for the OS on servers with 32GB+ RAM).
  • Run the following to verify current configuration via T-SQL:
EXEC sp_configure 'max server memory';

Step 5: Restore From a Known Good Backup if Corruption Persists

  • If DBCC CHECKDB reports unrepairable corruption tied to this error, restore the database from the last verified clean backup.
  • Use RESTORE DATABASE [YourDatabaseName] FROM DISK = 'BackupPath.bak' WITH REPLACE, RECOVERY;
  • Validate backup integrity beforehand using RESTORE VERIFYONLY FROM DISK = 'BackupPath.bak';

Alternative or Verification Steps

  • After applying fixes, monitor the SQL Server Error Log (ERRORLOG file in the Log directory) for 24-48 hours to confirm Error 8991 does not recur during peak load.
  • If the issue persists despite clean DBCC CHECKDB results and verified hardware, enable trace flag 3042 temporarily and capture a memory dump during the next occurrence for deeper analysis via Microsoft Support diagnostics.