How does disk caching work?

Jim C. Nasby jim at
Fri Apr 16 09:38:50 PDT 2004

Is there a document anywhere that describes in detail how FreeBSD
handles disk caching? I've read Matt Dillon's description of the VM
system, but it deals mostly with programs, other than vague statements
such as 'FreeBSD uses all available memory for disk caching'.

I think I know how caching memory mapped IO works for the most part,
since it should be treated just like program data, but what about files
that aren't memory mapped? What impact is there as pages move from
active to inactive to cache to free? What role do wired and buffer pages
Jim C. Nasby, Database Consultant                  jim at
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