How to fix Error 895 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Hybrid Buffer Pool Error 895

Why This Error Happens

Error 895 occurs when SQL Server 2025’s Hybrid Buffer Pool feature calculates the number of hash pages required to map Persistent Memory (PMEM) and encounters a configuration mismatch between the DAX-formatted volume size and the available non-volatile memory. This typically happens when the Persistent Memory volume is improperly aligned, the PMEM hardware is not correctly initialized in App Direct mode, or the underlying NVDIMM/Optane device drivers on Windows Server 2025 are outdated. Additionally, this message can surface as informational noise rather than a true failure if SQL Server is simply logging its internal memory-mapping proposal during startup diagnostics.

Step-by-Step Resolution Guide

Step 1: Verify Persistent Memory Hardware Configuration

  • Open Windows Admin Center or ipmctl show -dimm to confirm NVDIMMs are recognized and healthy.
  • Ensure the PMEM modules are configured in App Direct Mode (not Memory Mode), since Hybrid Buffer Pool only supports App Direct.
  • Run Get-PmemDisk in PowerShell to confirm the disk is visible to the OS.

Step 2: Validate the DAX Volume Formatting

  • Confirm the volume is formatted with DAX (Direct Access) enabled using:
    Get-Volume | Where-Object FileSystem -eq "NTFS" | Select DriveLetter, DedupMode
  • Reformat if necessary using:
    Format-Volume -DriveLetter F -FileSystem NTFS -DaxMode Enabled
  • Ensure the volume is not nested inside Storage Spaces Direct without proper PMEM passthrough.

Step 3: Update Persistent Memory and Storage Drivers

  • Navigate to Device Manager > Persistent Memory Controller.
  • Update drivers to the latest version certified for Windows Server 2025 (check vendor site, e.g., Intel Optane PMem drivers).
  • Reboot the server after driver installation to ensure proper DAX re-initialization.

Step 4: Reconfigure SQL Server Hybrid Buffer Pool Settings

  • Open SQL Server Configuration Manager and stop the SQL Server service.
  • Edit the startup parameter or use ALTER SERVER CONFIGURATION SET MEMORY_OPTIMIZED_HYBRID_BUFFER_POOL = ON;
  • Restart the SQL Server instance and monitor the SQL Error Log for Event ID 895 recurrence.

Alternative or Verification Steps

  • Check the SQL Server Error Log (ERRORLOG file in C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\) to confirm the hash page proposal completes without subsequent failure messages.
  • If the error persists, disable Hybrid Buffer Pool entirely via ALTER SERVER CONFIGURATION SET MEMORY_OPTIMIZED_HYBRID_BUFFER_POOL = OFF; as a temporary workaround while hardware diagnostics are performed.