Synchronizing Buffer Flushing

When shared memory is first initialized, all buffers are empty. As processing occurs, data pages are read from disk into the buffers, and user threads begin to modify these pages.

Ensuring That Physical-Log Buffers Are Flushed First

When page cleaning is initiated on the shared-memory buffer pool, the page-cleaner thread must coordinate the flushing so that the physical-log buffer is flushed first. Time-stamp comparison determines the order.

The database server stores a time stamp each time that the physical-log buffer is flushed. If a page-cleaner thread needs to flush a page in a shared-memory buffer, the page cleaner compares the time stamp in the modified buffer with the time stamp that indicates the point when the physical-log buffer was last flushed.

If the time stamp on the page in the buffer pool is equal to or more recent than the time stamp for the physical-log buffer flush, the before-image of this page conceivably could be contained in the physical-log buffer. In this case, the physical-log buffer must be flushed before the shared-memory buffer pages are flushed.

Flushing the Shared-Memory Pool Buffer

After the physical-log buffer is flushed, the user thread updates the time stamp in shared memory that describes the most-recent physical-log buffer flush. The specific page in the shared-memory buffer pool that is marked for flushing is now flushed. The number of modified buffers in the queue is compared to the value of LRU_MIN_DIRTY. If the number of modified buffers is greater than the value represented by LRU_MIN_DIRTY, another page buffer is marked for flushing. The time-stamp comparison is repeated. If required, the physical-log buffer is flushed again.

When no more buffer flushing is required, the page-cleaner threads sleep until buffer flushing is required again, and they are awakened to do the work. (For more information, refer to Sleep Queues.) You can tune the page-cleaning parameters (LRU_MIN_DIRTY and LRU_MAX_DIRTY) to influence the frequency of buffer flushing. For a description of how these parameters determine when page cleaning begins and ends, refer to LRU Queues.