The fsflush process writes modified pages to disk at regular intervals. The fsflush process scans through physical memory looking for dirty pages. When it finds one, it initiates a write (or putpage) operation on that page.
The fsflush process is launched by default every 5 seconds and looks for pages that have been modified (the modified bit is set in the page structure) more than 30 seconds ago. If a page has been modified, then a page-out is scheduled for that page, but without the free flag so the page remains in memory. The fsflush daemon flushes both data pages and inodes by default. Table 13-15 describes the parameters that affect the behavior of fsflush.