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Path: euryale.cc.adfa.oz.au!newshost.anu.edu.au!harbinger.cc.monash.edu.au!news.mel.connect.com.au!munnari.OZ.AU!news.hawaii.edu!news.uoregon.edu!news.sol.net!spool.mu.edu!howland.reston.ans.net!swrinde!newsfeed.internetmci.com!csn!nntp-xfer-1.csn.net!csn!nntp-xfer-2.csn.net!news.boulder.noaa.gov!news From: Sean Kelly <kelly@fsl.noaa.gov> Newsgroups: comp.unix.bsd.freebsd.misc Subject: Re: MFS - Why? Date: Mon, 10 Jun 1996 11:13:48 -0600 Organization: NOAA Forecast Systems Laboratory Lines: 29 Message-ID: <31BC57CC.36A2@fsl.noaa.gov> References: <4ogp99$kfu@nntp5.u.washington.edu> <ABgvNUrSgM@qsar.chem.msu.su> NNTP-Posting-Host: emu.fsl.noaa.gov Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Mailer: Mozilla 3.0b4 (X11; I; HP-UX B.10.01 9000/715) Eugene Radchenko wrote: > I guess I was not sufficiently clear in the original posting. If we have > merged disk cache/VM buffer and we open a file and write something to it, > then the data end up into that same buffer area as they would if we create > a file on the memory filesystem (with a difference that they could be > 'paged out' to normal filesystem if someone needs heaps of memory while the > MFS data could not). So what is the point? Good point. But I can think of another good reason to use MFS: if you've got multiple processes using mmap() to map files into their addresses spaces so that you can use shared memory as a form of IPC, then it's a heck of a lot better to mmap() a file that lives on an MFS filesystem than one that lives on a a real disk. As you make changes to the address space of the mmap()'d file, the kernel will also reflect those changes in the filesystem. If it's a real disk, then you're doing disk I/O. If it's MFS, then you're not! You save on the initial paging of the memory as well. For more details, see McKusick et al, _The Design and Implementation of the 4.4BSD Operating System_, page 139. -- Sean Kelly NOAA Forecast Systems Laboratory kelly@fsl.noaa.gov Boulder Colorado USA http://www-sdd.fsl.noaa.gov/~kelly/