This topic describes the role of the write -ahead transaction log in recording data modification to disk. For basic information about transaction logs, see Introduction to Transaction Logs . SQL Server uses a write-ahead log (WAL), which guarantees that no data modifications are written to disk before the associated log record is written to disk. This maintains the ACID properties for a transaction. For more information about transactions and ACID properties, see Transactions (Database Engine) . To understand how the write-ahead log works, it is important for you to know how modified data is written to disk. 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 made to the copy of the page in the buffer cache. The modification is not written to disk until a checkpoint occurs in the database, or the modificati...
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