SQL Server Log Truncation

Truncating the transaction log is a logical operation, just marking a portion of the log as ' no longer needed ' so you can reuse this space, truncation is not a physical operation, does not reduce the size of the file on disk,

To reduce the physical size, you must shrink.

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Sometimes a backup can not truncate the log:

This is because the user opens a transaction and forgets to open it, causing it (the transaction will not be committed and will not be rolled back, it will also be an active transaction)

SQL Server Log Truncation