How to fix Error 5848 on Windows Server 2025 / Windows 11

by

Troubleshooting Guide: SQL Server Error 5848 — Hot-Added CPU Notification

Why This Error Happens

This message is triggered when the underlying hardware or hypervisor performs a hot-add CPU operation on a system where SQL Server is running, which SQL Server detects and logs via its NUMA-aware scheduler. It is not actually an error condition but rather an informational event (Event ID 5848) confirming that a new physical CPU was dynamically added to a specific NUMA node and mapped to a logical CPU ID. This typically occurs in virtualized environments (Hyper-V, VMware) or on physical servers that support hot-plug CPU hardware, especially when a VM administrator increases vCPU allocation while the guest OS and SQL Server instance remain online.

Step-by-Step Resolution Guide

Step 1: Confirm the Message Is Informational, Not an Error

  • Check the SQL Server Error Log or Windows Event Viewer (Application log, source “MSSQLSERVER”) for Event ID 5848.
  • Verify the full message text includes “This is an informational message only. No user action is required.” — this confirms no corrective action is needed.
  • Cross-reference the timestamp with any recent hardware/VM configuration changes (e.g., vCPU hot-add performed by a hypervisor admin).

Step 2: Validate Hot-Add CPU Was Intentional

  • Open Hyper-V Manager or your virtualization platform’s console and check the VM settings under Processor configuration to confirm whether “Enable Processor compatibility” or hot-add CPU was deliberately enabled.
  • On Windows Server 2025 host, run the following in PowerShell to confirm current vCPU count matches expected allocation:
  Get-VM -Name "<VMName>" | Select ProcessorCount
  • If the hot-add was unauthorized or unexpected, coordinate with your infrastructure/virtualization team to confirm change management records.

Step 3: Verify SQL Server Recognizes the New CPU Correctly

  • Run the following T-SQL query to confirm SQL Server has updated its scheduler count and NUMA node mapping:
  SELECT cpu_id, scheduler_id, parent_node_id, status
  FROM sys.dm_os_schedulers
  WHERE status = 'VISIBLE ONLINE';
  • Compare the output against sys.dm_os_sys_info to ensure cpu_count matches the expected total after the hot-add:
  SELECT cpu_count, hyperthread_ratio, socket_count, numa_node_count
  FROM sys.dm_os_sys_info;

Step 4: Restart SQL Server Only If NUMA Mapping Appears Inconsistent

  • If sys.dm_os_schedulers or sys.dm_os_sys_info shows mismatched or orphaned NUMA node data, schedule a restart of the SQL Server service during a maintenance window:
  net stop MSSQLSERVER
  net start MSSQLSERVER
  • This forces SQL Server to fully re-enumerate all CPUs and NUMA nodes from scratch rather than relying on the dynamic hot-add detection logic.

Alternative or Verification Steps

  • Verify the fix: Re-run SELECT cpu_count, numa_node_count FROM sys.dm_os_sys_info; after any restart and confirm the values match the host’s current vCPU/NUMA configuration reported in Task Manager (Performance tab) or Get-VM output.
  • Alternative: If hot-add CPU events are frequent and generating log noise, consider disabling CPU hot-add in the VM’s hardware configuration (if dynamic scaling isn’t required) to reduce unnecessary Event ID 5848 entries in the SQL Server Error Log.