Physical logging is the process of storing the pages that the database server is going to change before the changed pages are actually recorded. Before the database server modifies certain pages in the shared-memory buffer pool, it stores an unmodified copy of the page (called a before-image) in the physical-log buffer in shared memory.
The physical log is a set of contiguous disk pages where the database server stores before-images.
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. Once a checkpoint occurs, the database server empties the physical log (except in the special circumstances explained in Limit to the Size of the Physical Log). For more information on checkpoints, see Checkpoints.