Changing the Wash Area for a Memory Pool

A portion of each pool is configured as the wash area.

When SAP ASE must read a buffer into cache, it places the buffer either:

At the LRU (least recently used) end of each memory pool, in a cache with strict LRU policy, or,
At the victim pointer, in a cache with relaxed LRU policy. If the recently used bit of buffer at the victim marker is set, the victim pointer is moved to the next buffer in the pool.

After dirty pages (pages that have been changed in cache) pass the wash marker and enter the wash area, SAP ASE starts an asynchronous I/O on the page. When the write completes, the page is marked clean and remains available in the cache.

The space in the wash area must be large enough so that the I/O on the buffer can complete before the page needs to be replaced. This figure illustrates how the wash area of a buffer pool works with a strict and relaxed LRU cache.

Wash area of a buffer pool
Graphic describing how a wash area works in a strict LRU cache compared to a relaxed LRU cache. The strict LRU cache is shown as a bar that is divided into pages. When the page with the dirty read passes the wash area, it is written to disk. The relaxed LRU cache is described as a clock. When the victim pointer passes the first dirty read, this page is written to disk.

Graphic describing how a wash area works in a strict LRU cache compared to a relaxed LRU cache. The strict LRU cache is shown as a bar that is divided into pages. When the page with the dirty read passes the wash area, it is written to disk. The relaxed LRU cache is described as a clock. When the victim pointer passes the first dirty read, this page is written to disk.

By default, the size of the wash area for a memory pool is configured as follows:

If the pool size is less than 300MB, the default wash size is 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 wash size is 10 buffers. The maximum size of the wash area is 80 percent of the pool size. You must specify the pool size and wash size for all pools larger than 2KB

A buffer is a block of pages that matches the I/O size for the pool. Each buffer is treated as a unit: all pages in the buffer are read into cache, written to disk, and aged in the cache as a unit. For the size of the block, multiply the number of buffers by the pool size—for a 2KB pool, 256 buffers equals 512KB; for a 16KB pool, 256 buffers equals 4096KB.

For example, if you configure a 16K pool with 1MB of space, the pool has 64 buffers; 20 percent of 64 is 12.8. This is rounded down to 12 buffers, or 192K, is the size of the block that is allocated to the wash area.

When the Wash Area Is Too Small
If the wash area is too small for the usage in a buffer pool, operations that need a clean buffer may have to wait for I/O to complete on the dirty buffer at the LRU end of the pool or at the victim marker. This is called a dirty buffer grab, and it can seriously impact performance.
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.
Setting the Housekeeper to Avoid Washes for Cache
You can use the HK ignore cache option of the cache status parameter to specify that you do not want the housekeeper to perform a wash on a particular cache.

Parent topic: Configuring Data Caches

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