How to fix Error 5850 on Windows Server 2025 / Windows 11

by

Troubleshooting Guide: SQL Server Error 5850 — Online CPU Addition Blocked by Soft-NUMA Configuration

Why This Error Happens

This error occurs because SQL Server 2025 detected a manually configured soft-NUMA setup (defined via registry keys or startup trace flags) at the time the instance started, and this static configuration explicitly maps CPUs to NUMA nodes. When Windows Server 2025 or Windows 11 hot-adds new virtual or physical CPUs to the host, SQL Server cannot dynamically re-map these new logical processors into the existing soft-NUMA node structure without a restart. Essentially, the soft-NUMA partitioning you defined is treated as fixed at startup, and the engine lacks the ability to reconcile that fixed map with newly available CPU resources while running.

Step-by-Step Resolution Guide

Step 1: Confirm the Current Soft-NUMA Configuration

  • Open Registry Editor (regedit) and navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\<InstanceID>\MSSQLServer\NodeConfiguration.
  • Review the CPUMask or CPUAffinityMask values under each NodeX key to confirm manual NUMA partitioning is active.
  • Alternatively, query SQL Server directly:
  SELECT * FROM sys.dm_os_nodes;

Step 2: Validate the New CPU Resources at the OS Level

  • Open Task Manager or System Information (msinfo32) on Windows Server 2025/Windows 11 to confirm the hot-added CPUs are recognized by the operating system.
  • Run the following PowerShell command to verify logical processor count:
  Get-WmiObject -Class Win32_ComputerSystem | Select-Object NumberOfLogicalProcessors
  • If the OS does not reflect the new CPUs, resolve the hardware/hypervisor-level hot-add issue before proceeding further.

Step 3: Restart the SQL Server Service to Apply the New CPU Mapping

  • Open SQL Server Configuration Manager.
  • Right-click the relevant SQL Server instance and select Restart.
  • This forces SQL Server to re-read the soft-NUMA configuration and correctly map all currently available CPUs—including the newly added ones—into NUMA nodes at startup.

Step 4: Reconfigure or Remove Manual Soft-NUMA Mapping (If Restart Is Not Desired Long-Term)

  • If frequent hot-add operations are expected, consider removing the manual NodeConfiguration registry entries to allow SQL Server to use automatic soft-NUMA detection instead.
  • Back up the registry key before deletion:
  reg export "HKLM\SOFTWARE\Microsoft\Microsoft SQL Server\<InstanceID>\MSSQLServer\NodeConfiguration" C:\Backup\NodeConfig_Backup.reg
  • Delete the NodeConfiguration key, then restart the SQL Server service so it auto-detects NUMA boundaries based on current hardware topology.

Alternative or Verification Steps

  • After restarting the service, run SELECT cpu_count FROM sys.dm_os_sys_info; to confirm SQL Server now reports the updated total CPU count matching the OS.
  • If a restart is not immediately feasible in production, document the pending CPU addition and schedule a maintenance window, as no online workaround exists—a full SQL Server service restart is required to apply new CPU resources under an active soft-NUMA configuration.