Date: Fri, 16 Apr 2004 17:12:11 -0500 From: "Jim C. Nasby" <jim@nasby.net> To: Igor Shmukler <shmukler@mail.ru> Cc: freebsd-performance@freebsd.org Subject: Re: How does disk caching work? Message-ID: <20040416221211.GM87362@nasby.net> In-Reply-To: <E1BEbKR-000ISM-00.shmukler-mail-ru@f7.mail.ru> References: <20040416163845.GG87362@nasby.net> <E1BEbKR-000ISM-00.shmukler-mail-ru@f7.mail.ru>
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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 Windows: "Where do you want to go today?" Linux: "Where do you want to go tomorrow?" FreeBSD: "Are you guys coming, or what?"
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