Avamar: SQL Plugin Log Truncation Does Not Shrink SQL Transaction Log Files

Avamar SQL Plugin Backup - Observed Behavior

Administrators may notice the following during or after an Avamar SQL Plugin backup:

• The SQL transaction log is truncated, but its physical file size does not decrease.
• Despite successful backup completion, the log file remains at its pre‑backup size.
• Expectations that the backup process will automatically shrink the log file are not met.

Misunderstanding of SQL Transaction Log Management

The issue originates from the incorrect assumption that the Avamar SQL Plug‑in truncates and shrinks SQL transaction log files during a backup. In reality, Avamar only signals SQL that a full backup has occurred, allowing SQL to truncate the log. It does not perform any log‑shrink operation.

• SQL truncation marks inactive Virtual Log Files (VLFs) as reusable, but the physical file size remains unchanged.
• Microsoft SQL Server’s default logic never reduces the physical log file size after truncation.
• Log growth parameters (initial size, growth increment, maximum size) control file expansion, not reduction
• DBAs often expect automatic shrinking, leading to confusion when log files retain their original size.

Impact of VLF Behavior

Each VLF is the smallest unit that can be marked active or inactive. If any VLF contains required log records, the entire VLF stays active, preventing reuse of that portion of the file. Truncation only frees completely inactive VLFs; it does not collapse the file.

Typical Administrative Assumptions

• Assuming a backup‑initiated truncation will also shrink the log file.
• Relying on Avamar to manage log size without a separate shrink routine.
• Overlooking the need for a scheduled DBCC SHRINKFILE or Management Console shrink task.

How Avamar SQL Plugin Handles Transaction Log Files

Avamar’s SQL plug‑in truncates the transaction log after a backup but does not shrink the physical log file. DBAs must shrink the log manually if a smaller file size is required.

Step‑by‑Step Procedure to Shrink SQL Transaction Logs

• Confirm that the most recent Avamar backup completed successfully.
• Verify that log truncation occurred (the log is marked reusable).
• Perform a manual shrink operation using SQL Server Management Studio (SSMS) or a T‑SQL script.
• Schedule regular shrink tasks if ongoing size reduction is needed.

For additional information regarding shrinking a Microsoft SQL Log file, see:

• Shrinking the Transaction Log (External Link)
• DBCC SHRINKFILE (Transact-SQL) (External Link)