Wash Percentage and sp_poolconfig

The default value for the wash size is computed as follows

  • If the pool size is less than 300MB, the default wash size is set to 20 percent of the buffers in the pool

  • If the pool size is greater than 300MB, the default wash size is 20 percent of the number of buffers in 300MB

The minimum setting for the wash size is 10 buffers, and the maximum setting is 80 percent of the size of the pool.

Each memory pool contains a wash area at the least recently used (LRU) end of the chain of buffers in that pool. Once dirty pages (pages that have been changed while in cache) move into the wash area, the SAP ASE server initiates asynchronous writes on these pages. The wash area must be large enough so that pages can be written to disk before they reach the LRU end of the pool. Performance suffers when the SAP ASE server needs to wait for clean buffers.

The default percentage, placing 20 percent of the buffers in the wash area, is sufficient for most applications. If you are using an extremely large memory pool, and your applications have a very high data modification rate, you may want to increase the size to 1 or 2 percent of the pool. Run sp_sysmon to look for recommendations, or contact Sybase Technical Support for more information about choosing an effective wash size.