Unbuffered transaction logging

If transactions are made against a database that uses unbuffered logging, the records in the logical-log buffer are guaranteed to be written to disk during commit processing. When control returns to the application after the COMMIT statement (and before the PREPARE statement for distributed transactions), the logical-log records are on the disk. The database server flushes the records as soon as any transaction in the buffer is committed (that is, a commit record is written to the logical-log buffer).

When the database server flushes the buffer, only the used pages are written to disk. Used pages include pages that are only partially full, however, so some space is wasted. For this reason, the logical-log files on disk fill up faster than if all the databases on the same database server use buffered logging.

Unbuffered logging is the best choice for most databases because it guarantees that all committed transactions can be recovered. In the event of a failure, only uncommitted transactions at the time of the failure are lost. However, with unbuffered logging, the database server flushes the logical-log buffer to disk more frequently, and the buffer contains many more partially full pages, so it fills the logical log faster than buffered logging does.

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