Manage your buffer pools

To manage your buffer pools efficiently, we must consider the factors that affect the buffer pool I/O operations and also the statistics associated with the buffer pools.

• If a page containing the required data is not found in the buffer pool, it is read in synchronously to an available buffer from its DASD page set.
• Whenever a page is updated, it is put on an internal queue of pages to be (potentially) written out to DASD. This means that the buffer used by that page is unavailable for use by any other page until the buffer has been written to DASD.
• If the number of pages queued to be written to DASD exceeds 85% of the total number of buffers in the pool, an asynchronous write processor is started to put the buffers to DASD.

  Similarly, if the number of buffers available for page get requests become less than 15% of the total number of buffers in the pool, the asynchronous write processor is started to perform the write I/O operations.

  The write processor stops when the number of pages queued to be written to DASD has fallen to 75% of the total number of buffers in the pool.

• If the number of pages queued for writing to DASD exceeds 95% of the total number of buffers in the pool, all updates result in a synchronous write of the page to DASD.

  Similarly, if the number of buffers available for page get requests becomes less than 5% of the total number of buffers in the pool, all updates result in a synchronous write of the page to DASD.

• If the number of buffers available for page get requests ever reaches zero, a transaction that encounters this condition is suspended until the asynchronous write processor has finished.
• If a page is frequently updated, the page spends most of its time on the queue of pages waiting to be written to DASD. Because this queue is in least recently used order, it is possible that a frequently updated page placed on this least recently used queue is never written out to DASD. For this reason, at the time of update, if the page is found to have been waiting on the write operation to DASD queue for at least two checkpoints, it is synchronously written to DASD. Updating occurs at checkpoint time and is suspended until the asynchronous write processor has finished.

  The aim of this algorithm is to maximize the time pages spend in buffer pool memory while allowing the system to function if the system load puts the buffer pool usage under stress.

Parent topic: Buffer manager data records