Microsoft® SQL Server™, like many relational databases, uses a write-ahead log. A write-ahead log ensures that no data modifications are written to disk before the associated log record.
SQL Server maintains a buffer cache into which it reads data pages when data must be retrieved. Data modifications are not made directly to disk, but are instead made to the copy of the page in the buffer cache. The modification is not written to disk until the lazywriter process schedules a write for the page. Writing a modified data page from the buffer cache to disk is called flushing the page. A page which has been modified in the cache but is not yet written to disk is called a dirty page.
At the time a modification is made to a page in the buffer, a log record is built in the log cache recording the modification. This log record must be written to disk before the associated dirty page is flushed from the buffer cache to disk. If the dirty page were flushed before the log record, it would create a modification on disk that could not be rolled back if the server failed before the log record were written to disk. SQL Server has logic that prevents a dirty page from being flushed before the associated log record. Because log records are always written ahead of the associated data pages, the log is called a write-ahead log.
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