The Adaptive Server data cache is managed on a most recently used/least recently used (MRU/LRU) basis. As pages in the cache age, they enter a wash area, where any dirty pages (pages that have been modified while in memory) are written to disk. There are some exceptions to this:
Caches configured with relaxed LRU replacement policy use the wash section as described above, but are not maintained on an MRU/LRU basis.
Typically, pages in the wash section are clean, i.e. the I/O on these pages have been completed. When a task or query wants to grab a page from LRU end it expects the page to be clean. If not, the query has to wait for the I/O to complete on the page before it can be grabbed which impairs performance.
A special strategy ages out index pages and OAM pages more slowly than data pages. These pages are accessed frequently in certain applications and keeping them in cache can significantly reduce disk reads.
See the System Administration Guide for more information.
Adaptive Server may choose to use the LRU cache replacement strategy that does not flush other pages out of the cache with pages that are used only once for an entire query.
The checkpoint process ensures that if Adaptive Server needs to be restarted, the recovery process can be completed in a reasonable period of time.
When the checkpoint process estimates that the number of changes to a database will take longer to recover than the configured value of the recovery interval configuration parameter, it traverses the cache, writing dirty pages to disk.
Recovery uses only the default data cache making it faster.
The housekeeper task writes dirty pages to disk when idle time is available between user processes.