Re: How does disk caching work?

From: Jim C. Nasby (jim_at_nasby.net)
Date: 04/17/04

  • Next message: Aaron Seelye: "Re: command piped into bzip not using all available CPU" 
    Date: Fri, 16 Apr 2004 17:12:11 -0500
To: Igor Shmukler <shmukler@mail.ru>

    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@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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