Re[2]: How does disk caching work?
From: Q?=Igor ShmuklerQ=20?= (shmukler_at_mail.ru)
Date: 04/17/04
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To: Q?=Jim C.NasbyQ=20?= <jim@nasby.net> Date: Sat, 17 Apr 2004 02:19:53 +0400
> > > 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?
Read/write/execute require mmap as far as I know. What type of access do you want to perform without the mmap?
Not that you asked, but I strongly suggest reading AST's book than Design of 44BSD.
> > 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.
Ok, I thought that there was an unknown concept of buffer pages. :)
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