How to fix Error 8915 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 8915 — Mismatched Mixed Extent Count

Why This Error Happens

Error 8915 occurs when the SQL Server storage engine detects that the actual number of mixed extents allocated to a database file does not match the count recorded in the file’s header (PFS/GAM page metadata). This mismatch is typically caused by physical disk corruption, improper shutdowns, storage subsystem failures (SAN/RAID controller issues), or interrupted write operations during a crash. It is a DBCC CHECKDB-reported allocation-level consistency error indicating the database’s internal bookkeeping is out of sync with the actual page allocations on disk.

Step-by-Step Resolution Guide

Step 1: Confirm and Document the Error Scope

  • Run DBCC CHECKDB('YourDatabaseName') WITH NO_INFOMSGS, ALL_ERRORMSGS; in SSMS to capture the full list of errors, not just 8915.
  • Note the File ID (%d) and the reported vs. expected extent counts from the error output.
  • Check the SQL Server Error Log (C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG) for correlating I/O or disk warnings (e.g., Event ID 823, 824, 825).

Step 2: Rule Out Underlying Storage Problems

  • Open Windows Event Viewer (eventvwr.msc) and check the System log for Disk, ntfs, or storport errors around the time the corruption appeared.
  • Run chkdsk D:\ /f /r (replace D: with the volume hosting your .mdf/.ndf files) during a maintenance window to repair file system-level issues.
  • If using virtualized or SAN storage, validate the underlying LUN health via your storage vendor’s diagnostic tool before proceeding further.

Step 3: Restore from a Known Good Backup (Preferred Fix)

  • Take a tail-log backup if the database is still accessible: BACKUP LOG YourDatabaseName TO DISK = 'D:\Backups\YourDB_tail.trn';
  • Restore the last clean full backup plus subsequent log backups up to the point before corruption: RESTORE DATABASE YourDatabaseName FROM DISK = 'D:\Backups\YourDB_Full.bak' WITH NORECOVERY;
  • Apply each transaction log backup sequentially using RESTORE LOG ... WITH NORECOVERY, finishing with WITH RECOVERY on the last one.

Step 4: Run Repair as a Last Resort (If No Valid Backup Exists)

  • Set the database to single-user mode: ALTER DATABASE YourDatabaseName SET SINGLE_USER WITH ROLLBACK IMMEDIATE;
  • Execute DBCC CHECKDB('YourDatabaseName', REPAIR_ALLOW_DATA_LOSS) WITH NO_INFOMSGS; — note this can delete corrupted allocation units.
  • Revert to multi-user mode: ALTER DATABASE YourDatabaseName SET MULTI_USER;

Step 5: Re-verify Allocation Integrity

  • Re-run DBCC CHECKDB with ALL_ERRORMSGS to confirm Error 8915 and related errors no longer appear.
  • Rebuild affected indexes if REPAIRALLOWDATA_LOSS dropped any pages: ALTER INDEX ALL ON YourTable REBUILD;

Alternative or Verification Steps

  • Verify the fix: Run DBCC CHECKDB('YourDatabaseName') WITH NO_INFOMSGS, ALL_ERRORMSGS; a final time — a clean run with zero allocation errors confirms resolution.
  • Alternative approach: If corruption recurs frequently on the same volume, migrate the database files to a new disk/LUN via ALTER DATABASE ... MODIFY FILE and decommission the suspect storage to rule out recurring hardware failure.