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Received: by minnie.vk1xwt.ampr.org with NNTP id AA5135 ; Tue, 22 Dec 92 03:01:14 EST Path: sserve!manuel.anu.edu.au!munnari.oz.au!spool.mu.edu!yale.edu!qt.cs.utexas.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!menudo.uh.edu!wjin From: wjin@cs.uh.edu (W. Woody Jin) Newsgroups: comp.unix.bsd Subject: Re: Dumb Question: Why 512 byte block? Date: 18 Dec 1992 18:59:50 GMT Organization: University of Houston Lines: 48 Message-ID: <1gt736INNjje@menudo.uh.edu> References: <1992Dec18.005050.20594@decuac.dec.com> <1992Dec18.030833.7395@fcom.cc.utah.edu> NNTP-Posting-Host: rodin.cs.uh.edu In article <1992Dec18.030833.7395@fcom.cc.utah.edu> terry@cs.weber.edu (A Wizard of Earth C) writes: >In article <1992Dec18.005050.20594@decuac.dec.com>, darryl@vfofu1.dco.dec.com (Darryl Wagoner) writes: >|> Why is everything in 1/2 k block instead the BSD standard of 1024 byte blocks? >|> Yes, I know there is a '-k' switch, but it seems to me it should be >|> the otherway around. > >Think of disk blocks as the curve-fitting algoritm they taught you when you >first learned integral calculus: the smaller your slices, the closer you >come to approximating the area under the curve. > >If I have a set of 6 512 byte files, I will use up 3K of disk for them; >similarly, if I had a blocking factor of 1K, I would use of 6K (since the >smallest fragment usable by a file is now 1K. > >If I have 6 1.5K files, this translates to 9K of disk(512B) or 12K of disk(1K). >Obviously, if I have 6 1.6K files, both blocking factors take up 512K. I did not think that Fast Unix File System works this way. If you have 6 1.5K files using 1K blocks, 6 1k blocks will take each 1K from each file, and 3 1k blocks will take 0.5k from each file. This was my understading from the famous FUFS paper. >The offset into the disk is a _block_offset_; what this means is that you >will start looking for data at the offet*blocking_size when given an address, >and that reads/write into kernel cache are done (usually) in block_size >increments. A device accessed this way is a blocked device. > >When you store a lot of little files on the disk, small blocks are wasteful >of disk space (for instance, 1024 empty files take 512K vs 1M of disk for >the smaller blocking factor). And choosing a disk block size is a performance issue rather than disk space issue. One should tune disk block size and fragment size to get the best performance. I thought that 386BSD uses this Fast Unix File System. -- ____ ____ ____ ____________________________________ (___) _________________ | | | | | | W. Woody Jin (wjin@cs.uh.edu) (o o) Moo.... | | | |__| | PhD Student. Research Asst. o=======\ / I'm a Cow Lover. | | | | Dept. of Computer Science / | ||O My wife was born \ |---| |--| | University of Houston ` ||'---|| in Cow year. Mooo \____/|__| |__| _______________________________^^ ^^_____________________