LOG TRUNCATION

Log truncation occurs at these points:
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>> At the completion of any BACKUP LOG statement.

>> Every time a checkpoint is processed, provided the database is using the simple recovery model. This includes both explicit checkpoints resulting from a CHECKPOINT statement and implicit checkpoints generated by the system.

>> The exception is that the log is not truncated if the checkpoint occurs when a BACKUP statement is still active. For more information about the interval between automatic checkpoints, see Checkpoints and the Active Portion of the Log..

>> Transaction logs are divided internally into sections called virtual log files. Virtual log files are the unit of truncation. When a transaction log is truncated, all log records before the start of the virtual log file containing the MinLSN are deleted.