The database server temporarily stores before-images of some of the modified disk pages in the physical-log buffer. If the before-image has been written to the physical-log buffer but not to the physical log on disk, the physical-log buffer must be flushed to disk before the modified page can be flushed to disk. This action is required for the fast-recovery feature.
Both the physical-log buffer and the physical log contribute toward maintaining the physical and logical consistency of the data. For information on physical logging, checkpoints, and fast recovery, refer to Physical Logging, Checkpoints, and Fast Recovery.
The following events cause the current physical-log buffer to flush:
The contents of the physical-log buffer must always be flushed to disk before any data buffers. This rule is required for the fast-recovery feature.
The database server uses only one of the two physical-log buffers at a time. This buffer is the current physical-log buffer. Before the database server flushes the current physical-log buffer to disk, it makes the other buffer the current buffer so that it can continue writing while the first buffer is being flushed.
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