To return all disk pages to their condition at the time
of the most recent checkpoint, the database server writes the before-images
stored in the physical log to shared memory and then back to disk.
Each before-image in the physical log contains the address of a page
that was updated after the checkpoint. When the database server writes
each before-image page in the physical log to shared memory and then
back to disk, changes to the database server data since the time of
the most recent checkpoint are undone. The database server is now
physically consistent. The following figure illustrates this step.Figure 1: Writing all remaining
before-images in the physical log back to disk
