How to fix Error 5266 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 5266 – RSVD Page Count Mismatch

Why This Error Happens

Error 5266 occurs when SQL Server’s DBCC CHECKDB (or CHECKALLOC) detects a discrepancy between the reserved (RSVD) page count recorded in the system catalog (sys.allocation_units) and the actual number of pages physically allocated to an object. This typically stems from corrupted allocation metadata (IAM/GAM/SGAM pages) caused by abrupt shutdowns, storage-level I/O failures, or a bug in internal page tracking during large bulk operations. It can also surface after improper VSS-based snapshot backups or SAN replication that captured an inconsistent state of the database files on Windows Server 2025 storage volumes.

Step-by-Step Resolution Guide

Step 1: Confirm the Scope of Corruption

  • Run DBCC CHECKDB('YourDatabaseName') WITH NO_INFOMSGS, ALL_ERRORMSGS; from SQL Server Management Studio (SSMS) or sqlcmd.
  • Review the output carefully to confirm Error 5266 is isolated to allocation metadata and not accompanied by other critical errors (e.g., 8928, 8944), which would indicate deeper page-level corruption.
  • Note the exact object ID and index ID referenced in the error message for targeted repair.

Step 2: Rule Out Storage and Hardware Issues

  • Check the Windows Event Viewer (eventvwr.msc) under Windows Logs > System for Disk or NTFS warnings correlating with the SQL error timestamp.
  • Run chkdsk C:\ /f /r (adjust drive letter to your data/log file path) during a maintenance window to validate underlying disk integrity on Windows Server 2025.
  • Verify that antivirus or VSS snapshot tools are not interfering with .mdf/.ldf files, and exclude the SQL Server data directory (commonly C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\DATA\) from real-time scanning.

Step 3: Attempt a Targeted Repair

  • If corruption is confirmed only in allocation pages, run: DBCC CHECKDB('YourDatabaseName', REPAIR_REBUILD);
  • If REPAIR_REBUILD does not resolve it, take a full backup first, then run the more aggressive option: DBCC CHECKDB('YourDatabaseName', REPAIR_ALLOW_DATA_LOSS);
  • Execute this only on a copy of the database if possible, since REPAIR_ALLOW_DATA_LOSS can delete pages to resolve allocation mismatches.

Step 4: Restore From a Clean Backup (If Repair Fails)

  • Locate your most recent verified-clean full backup, typically stored on your backup server or Azure Blob target.
  • Restore it using: RESTORE DATABASE YourDatabaseName FROM DISK = 'D:\Backups\YourDatabaseName.bak' WITH REPLACE;
  • Apply subsequent transaction log backups if point-in-time recovery is required, and re-run DBCC CHECKDB post-restore to confirm integrity.

Alternative or Verification Steps

  • After applying a fix, re-run DBCC CHECKDB('YourDatabaseName') WITH NO_INFOMSGS, ALL_ERRORMSGS; and confirm it returns “0 allocation errors and 0 consistency errors.”
  • If the error recurs on the same object after repair, consider rebuilding the affected index entirely using ALTER INDEX ALL ON YourTableName REBUILD; or migrating the table data to a new object via SELECT INTO as a last-resort alternative.