How to fix Error 850 on Windows Server 2025 / Windows 11

by

Why This Error Happens

This message is actually an informational event, not a failure—it confirms that SQL Server 2025 successfully enabled Lock Pages in Memory (LPIM) and allocated large pages for the Buffer Pool. It typically appears when the SQL Server service account has been granted the “Lock Pages in Memory” user right and the server is running Enterprise Edition with sufficient contiguous physical memory. However, if you’re troubleshooting this because of unexpected memory pressure or startup delays, the root cause is usually that large page allocation reserves memory in large contiguous blocks upfront, which can fail or delay startup if the OS cannot find enough contiguous physical RAM due to fragmentation.

Step-by-Step Resolution Guide

Step 1: Verify the Lock Pages in Memory Policy

  • Open secpol.msc or gpedit.msc on Windows Server 2025.
  • Navigate to Local Policies > User Rights Assignment > Lock pages in memory.
  • Confirm the SQL Server service account (e.g., NT Service\MSSQLSERVER or a domain service account) is listed.
  • If missing, add the account, then restart the SQL Server service for the policy to take effect.

Step 2: Check Physical Memory and Fragmentation

  • Run systeminfo in an elevated Command Prompt to confirm total physical RAM matches expectations.
  • Use Performance Monitor (perfmon) and add the counter Memory > Available MBytes to check for fragmentation issues during startup.
  • If memory is highly fragmented (common after long uptime or many application restarts), schedule a server reboot to consolidate contiguous memory blocks.

Step 3: Confirm Trace Flag and Startup Configuration

  • Open SQL Server Configuration Manager and check the Startup Parameters tab for the SQL Server service.
  • Ensure no conflicting trace flags (like -T834, which forces large page allocation for the buffer pool) are misconfigured for your hardware capacity.
  • If -T834 is enabled on a system with limited RAM, consider removing it to allow standard page allocation instead, reducing startup memory pressure.

Step 4: Review SQL Server Error Log for Context

  • Open the SQL Server Error Log via SSMS (Management > SQL Server Logs) or navigate to the log file directly at:
    C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG
  • Search for Event ID 850 entries and note the MB value reported.
  • Cross-reference this value against max server memory settings in sp_configure to ensure allocation aligns with your intended Buffer Pool size.

Alternative or Verification Steps

  • Run SELECT * FROM sys.dm_os_sys_info; in SSMS and check the sql_memory_model_desc column—it should return LOCK_PAGES if large page allocation succeeded correctly.
  • If startup times remain slow despite correct configuration, temporarily disable Lock Pages in Memory, restart the service, and confirm whether standard memory allocation resolves the delay before re-enabling it with adjusted max server memory limits.