Flush the physical-log buffer

The database server always flushes the contents of the physical-log buffer to disk before any data buffers.

The database server temporarily stores before-images of some of the modified disk pages in the physical-log buffer. If the before-image is written to the physical-log buffer but not to the physical log on disk, the server flushes the physical-log buffer to disk before flushing the modified page to disk.

The following events cause the active physical-log buffer to flush:
  • The active physical-log buffer becomes full.
  • A modified page in shared memory must be flushed, but the before-image is still in the active physical-log buffer.
  • A checkpoint occurs.

The database server uses only one of the two physical-log buffers at a time. This buffer is the active (or current) physical-log buffer. Before the database server flushes the active physical-log buffer to disk, it makes the other buffer the active physical-log buffer so that the server can continue writing to a buffer while the first buffer is being flushed.

Both the physical-log buffer and the physical log help maintain the physical and logical consistency of the data. For information about physical logging, checkpoints, and fast recovery, see Physical logging, checkpoints, and fast recovery.


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