How does disk caching work?

Jim C. Nasby jim at
Fri Apr 16 15:12:12 PDT 2004

On Sat, Apr 17, 2004 at 01:56:55AM +0400, "Igor Shmukler"  wrote:
> > 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'.
> Well, the statement is not vague. FreeBSD has a unified buffer cache. This means that ALL AVAILABLE 
> MEMORY IS A BUFFER CACHE for all device IO.
> > 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
> > play?
> If file is not memory mapped it is not in memory, is it? Where do you cache it? Maybe I am missing 
> somewhing? Do you maybe want to know about node caching?

What if the file isn't memory mapped? You can access a file without
mapping it into memory, right?

> When pages are rotated from active to inactive and then to cache buckets they is still retains vnode 
> references. Once it is in free queue, there is no way to put it back to cache. Association is lost.
> Wired pages are to pin memory. So that we do not get situation when fault handling code is paged out.
> I am not FreeBSD guru so I never heard of BUFFER pages. Is there such a concept?

I'm reffering to the 'Buf' column at the top of top. I remember reading
something about that being used to cache file descriptors before the
files are mapped into memory, but I'm not very clear on what is actually
Jim C. Nasby, Database Consultant                  jim at
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