How to fix Error 3221 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 3221 – ReadFileEx Incomplete Read

Why This Error Happens

Error 3221 occurs when SQL Server 2025 attempts to read a backup or database file via the ReadFileEx system function, but the operation returns fewer bytes than expected. This is typically caused by a corrupted or truncated backup file, an unstable storage connection (SAN/NAS/network share) interrupting the I/O operation, or antivirus/backup software locking the file mid-read. It can also occur if the file was still being written to by another process when SQL Server attempted access.

Step-by-Step Resolution Guide

Step 1: Verify File Integrity and Accessibility

  • Confirm the file path referenced in the error (%ls) exists and is not zero-byte or truncated.
  • Run CHECKSUM validation during restore using RESTORE VERIFYONLY FROM DISK = 'path\to\file.bak' to confirm the backup isn’t corrupted.
  • Check file properties in Windows Explorer or via PowerShell (Get-Item "D:\Backups\file.bak" | Select Length) to ensure the size matches the expected backup size.

Step 2: Rule Out Storage and Network Instability

  • If the file resides on a network share or SAN/NAS, test for packet loss or latency using ping and Test-NetConnection against the storage host.
  • Review Windows Event Viewer under Windows Logs > System for Disk, iSCSI, or NTFS warnings (Event ID 7, 11, or 51) occurring at the same timestamp as Error 3221.
  • Move the backup/data file locally to a fast local SSD temporarily and retry the operation to isolate whether storage is the bottleneck.

Step 3: Exclude Antivirus and Backup Software Interference

  • Add SQL Server data, log, and backup directories (e.g., D:\SQLData\, D:\SQLBackups\) to your antivirus exclusion list.
  • Disable third-party backup agents (e.g., Veeam, Commvault) temporarily during the SQL restore/backup window to confirm no file-locking conflict.
  • Restart the SQL Server service (net stop MSSQLSERVER then net start MSSQLSERVER) after making exclusion changes.

Step 4: Check Disk Health and File System Errors

  • Run chkdsk D: /f /r on the volume hosting the SQL files (requires a reboot if the volume is in use).
  • Review the disk’s SMART status using Get-PhysicalDisk | Get-StorageReliabilityCounter in PowerShell to check for pending sector failures.
  • If using a virtualized environment (Hyper-V/VMware), verify the virtual disk isn’t experiencing synthetic I/O timeouts in the hypervisor host logs.

Alternative or Verification Steps

  • Re-run the original operation (RESTORE DATABASE or backup job) and confirm Error 3221 no longer appears in the SQL Server Error Log (EXEC sp_readerrorlog).
  • If the error persists only on a specific file, attempt to restore from an earlier, known-good backup copy to confirm whether the issue is isolated to file corruption rather than environmental instability.