SQL Server Architecture - Logging and Transaction

Logging and Transaction
SQL Server ensures that any change to the data is ACID-compliant, i.e. it uses transactions to ensure that the database will always revert to a known consistent state on failure. Each transaction may consist of multiple SQL statements all of which will only make a permanent change to the database if the last statement in the transaction (a COMMIT statement) completes successfully. If the COMMIT successfully completes the transaction is safely on disk.

SQL Server implements transactions using a write-ahead log.

Any changes made to any page will update the in-memory cache of the page, simultaneously all the operations performed will be written to a log, along with the transaction ID which the operation was a part of. Each log entry is identified by an increasing Log Sequence Number (LSN) which is used to ensure that all changes are written to the data files. Also during a log restore it is used to check that no logs are duplicated or skipped. SQL Server requires that the log is written onto the disc before the data page is written back. It must also ensure that all operations in a transaction are written to the log before any COMMIT operation is reported as completed.

At a later point the server will checkpoint the database and ensure that all pages in the data files have the state of their contents synchronised to a point at or after the LSN that the checkpoint started. When completed the checkpoint marks that portion of the log file as complete and may free it (see Simple transaction logging vs Full transaction logging). This enables SQL Server to ensure integrity of the data, even if the system fails.

On failure the database log has to be replayed to ensure the data files are in a consistent state. All pages stored in the roll forward part of the log (not marked as completed) are rewritten to the database, when the end of the log is reached all open transactions are rolled back using the roll back portion of the log file.

The database engine usually checkpoints quite frequently. However, in a heavily loaded database this can have a significant performance impact. It is possible to reduce the frequency of checkpoints or disable them completely but the rollforward during a recovery will take much longer

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