A full database backup does not truncate the registry. It does not decrease the size of the transaction log file, nor does it affect a T-log backup. A full backup contains only the transaction log necessary so that, in the event of a restore, the database can be transactionally consistent with when the data read portion of the full backup was completed. Verification of how much transaction log a full backup includes reveals that log backups during full backups will not truncate the log.
That has to wait until the first log backup is finished. For routine log backups, it is recommended to keep the default selection, Truncate the transaction log by deleting inactive entries. It is important to note that a full backup does not truncate or delete transaction log data. The transaction log file works independently of the full backup file.
When you perform full backups and transaction backups at the same time, transaction log backups are cleaned only after the full backup is complete. If you just run a full backup, your transaction log will remain intact and will not be cleaned. Only a transaction log backup can clean the transaction file. Truncation does not mean a physical reduction in the size of the transaction log file; it means that the inactive part of the transaction log file is marked as reusable.
Therefore, for truncation, you have to make a full backup of the database or (easier and faster) temporarily change it to simple recovery mode. If there is no backup of the database transaction log, the transaction log file will grow continuously, without being truncated, until it runs out of free space. To prevent this from happening, it is recommended to back up the transaction logs several times a day and do a nightly backup. After weekly maintenance and reindexing, truncating the log is also recommended as maintenance makes it very large before starting the weekly routine again. When configuring the database with the simple recovery model, SQL Server's transaction log will be marked as inactive and automatically truncated after confirming active transactions.
When this happens, all VLFs marked as inactive will be removed from SQL Server's transaction log file and available for reuse. If there is a single active record in a VLF, however, it will be considered an active record and cannot be truncated.