When the Wash Area is Too Large

If the wash area in a pool is too large, the buffers move too quickly past the wash marker in cache, and an asynchronous write is started on any dirty buffers.

This is described in this figure:

Effects of making the wash area too large
Image shows the effect of having the wash area too large. When the buffer reaches the wash marker too quickly, Adaptive Server writes more often to disk than necessary, wasting I/O.

Image shows the effect of having the wash area too large. When the buffer reaches the wash marker too quickly, Adaptive Server writes more often to disk than necessary, wasting I/O.

The buffer is marked clean and remains in the wash area of the MRU/LRU chain until it reaches the LRU. If another query changes a page in the buffer, SAP ASE must perform additional I/O to write the buffer to disk again.

If sp_sysmon output shows a high percentage of buffers “Found in Wash” for a strict replacement policy cache, and there are no problems with dirty buffer grabs, try reducing the size of the wash area. See the Performance and Tuning Series: Monitoring SAP Adaptive Server with sp_sysmon.