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Description of the reasons that the SQL Server transaction log is not being truncated
Content provided by Microsoft
Listed below are some reasons why the transaction log fills up and never gets truncated, even though the "trunc. log on chkpnt" option is set to true and regular checkpoints are done during processing.
An unbounded update fills up the log; that is, there is no WHERE clause on the UPDATE statement. Since a transaction (either user declared or implicit) must be able to be entirely rolled back or committed as a whole, the log must be large enough to maintain all information for the transaction. Thus, even when truncating the log, it is possible to fill the log if it is too small for a single large transaction.
This situation can also occur if a WHERE clause is used, the table is very large, and the WHERE clause is too general, causing a large number of records to be affected. There is another case where this could happen: if multiple updates are performed that together affect enough records to fill the log before the checkpoint is done, the log can still fill up.