How to fix Error 3048 on Windows Server 2025 / Windows 11

by

Troubleshooting Guide: Error 3048 – BACKUP LOG WITH TRUNCATE_ONLY Fails on Mirrored Database

Why This Error Happens

This error occurs because BACKUP LOG WITH TRUNCATE_ONLY (and the equivalent NO_LOG option) was deprecated and is fundamentally incompatible with the full recovery model requirements of database mirroring. SQL Server 2025 enforces this restriction because truncating the transaction log without a proper backup chain would break the log stream that mirroring depends on to synchronize the principal and mirror databases. Essentially, the command conflicts with the continuous log-shipping mechanism that mirroring uses to keep both database copies in sync.

Step-by-Step Resolution Guide

Step 1: Verify the Mirroring Configuration Status

  • Open SQL Server Management Studio (SSMS) and connect to the instance running on Windows Server 2025.
  • Run the following query to confirm the database mirroring state:
  SELECT name, mirroring_state_desc FROM sys.database_mirroring WHERE database_id = DB_ID('YourDatabaseName');
  • Confirm mirroring_state_desc returns a value other than NULL (e.g., SYNCHRONIZED), which confirms mirroring is active.

Step 2: Replace TRUNCATE_ONLY with a Proper Log Backup

  • Abandon the deprecated TRUNCATE_ONLY syntax entirely, as it is not supported for mirrored databases by design.
  • Execute a standard transaction log backup instead to properly clear the log:
  BACKUP LOG [YourDatabaseName] TO DISK = 'C:\SQLBackups\YourDatabaseName_Log.trn';
  • Ensure the target directory (e.g., C:\SQLBackups\) exists and the SQL Server service account has write permissions to it.

Step 3: Schedule Regular Log Backups via SQL Agent

  • Open SQL Server Agent in SSMS and create a new Job under Jobs > New Job.
  • Configure a recurring schedule (e.g., every 15-30 minutes) to run the BACKUP LOG command from Step 2, preventing the log file from growing uncontrollably.
  • Verify the SQL Server Agent service is running in Services.msc on Windows Server 2025, as a stopped Agent service will prevent the schedule from executing.

Step 4: Temporarily Remove Mirroring (If Log Truncation is Still Required)

  • If you absolutely must break the log chain, you will need to pause or remove mirroring first using:
  ALTER DATABASE [YourDatabaseName] SET PARTNER OFF;
  • Perform your log maintenance, then reconfigure mirroring by re-establishing the endpoint and partner relationship once finished.

Alternative or Verification Steps

  • Verify the fix by checking the log file size via DBCC SQLPERF(LOGSPACE); after running the scheduled backup job to confirm the log is being truncated correctly.
  • If mirroring is no longer a business requirement, consider migrating to Always On Availability Groups, which offers more modern log management flexibility compared to legacy database mirroring.