The Transaction Log (SQL Server) | Transaction log truncation

 Log truncation frees space in the log file for reuse by the transaction log. You must regularly truncate your transaction log to keep it from filling the allotted space. Several factors can delay log truncation, so monitoring log size matters. Some operations can be minimally logged to reduce their impact on transaction log size.

Log truncation deletes inactive virtual log files (VLFs) from the logical transaction log of a SQL Server database, freeing space in the logical log for reuse by the Physical transaction log. If a transaction log is never truncated, it will eventually fill all the disk space allocated to physical log files.

To avoid running out of space, unless log truncation is delayed for some reason, truncation occurs automatically after the following events:

• Under the simple recovery model, after a checkpoint.
• Under the full recovery model or bulk-logged recovery model, if a checkpoint has occurred since the previous backup, truncation occurs after a log backup (unless it is a copy-only log backup).
• When you first create a database using the FULL recovery model, the transaction log will be reused as needed (similar to a SIMPLE recovery database), up until the time you create a full database backup.

For more information, see Factors that can delay log truncation, later in this topic.