How does disk caching work?

[Jim C. Nasby]

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
happening.
-- 
Jim C. Nasby, Database Consultant                  jim at nasby.net
Member: Triangle Fraternity, Sports Car Club of America
Give your computer some brain candy! www.distributed.net Team #1828

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