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Under normal circumstances, the space in the log file is regularly reused, with space being made available for reuse through a process called log truncation. When log truncation cannot occur for some reason, the log file will attempt to grow if autogrow is enabled and the file has not reached its maximum size. When a log file becomes full and can no longer grow, attempts to modify or add data to the database will produce an error: the database will database  essentially become becomes read-only.

When a database log file is full, it is important to identify what is preventing log truncation from occurring. Two common reasons that the log might not be truncating include:

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