Configuring buffer wash size

You can configure the wash area for each pool in each cache. If you set the wash size is set too high, Adaptive Server may perform unnecessary writes. If you set the wash area too small, Adaptive Server may not be able to find a clean buffer at the end of the buffer chain and may have to wait for I/O to complete before it can proceed. Generally, wash size defaults are correct and need to be adjusted only in large pools that have very high rates of data modification.

Adaptive Server allocates buffer pools in units of logical pages. For example, on a server using 2K logical pages, 8MB are allocated to the default data cache. By default this constitutes approximately 4096 buffers.

If you allocated the same 8MB for the default data cache on a server using a 16K logical page size, the default data cache is approximately 512 buffers. On a busy system, this small number of buffers might result in a buffer always being in the wash region, causing a slowdown for tasks requesting clean buffers.

In general, to obtain the same buffer management characteristics on larger page sizes as with 2K logical page sizes, you should scale the size of the caches to the larger page size. In other words, if you increase your logical page size by four times, your cache and pool sizes should be about four times larger as well.

Queries performing large I/O, extent- based reads and writes, and so on, benefit from the use of larger logical page sizes. However, as catalogs continue to be page-locked, there is greater contention and blocking at the page level on system catalogs.Row and column copying for DOL tables will result in a greater slowdown when used for wide columns. Memory allocation to support wide rows and wide columns will marginally slow the server.

See the System Administration Guide for more information.