Physical logging

Physical logging is the process of storing the pages that the database server is going to change before the changed pages are actually recorded on disk. Before the database server modifies certain pages in the shared-memory buffer pool, it stores before-images of the pages in the physical-log buffer in shared memory.

The database server maintains the before-image page in the physical-log buffer in shared memory for those pages until one or more page cleaners flush the pages to disk. The unmodified pages are available in case the database server fails or the backup procedure requires them to provide an accurate snapshot of the database server data. Fast recovery and database server backups use these snapshots.

The database server recycles the physical log at each checkpoint, except in the special circumstances. For more information about checkpoints, see Checkpoints.