Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site rlgvax.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!godot!harvard!seismo!rlgvax!guy From: guy@rlgvax.UUCP (Guy Harris) Newsgroups: net.periphs Subject: Re: caching disk controllers for Unix Message-ID: <326@rlgvax.UUCP> Date: Sat, 5-Jan-85 22:42:31 EST Article-I.D.: rlgvax.326 Posted: Sat Jan 5 22:42:31 1985 Date-Received: Mon, 7-Jan-85 02:16:59 EST References: <69@mot.UUCP> Organization: CCI Office Systems Group, Reston, VA Lines: 19 > the sector cachers say you gotta do it this way because of Unix' habit of > fragmenting a file all over the place.... > the latter seems contrary to the free list, which is fairly integral to > the current Unix kernel's view of the file system (no reason why there > couldn't be another, I suppose). Can you say "4.2BSD"? I thought you could! I suspect several other variants of the UNIX file system have also been done that use a bit map rather than a free list; I suspect most other operating systems use bit maps as well. The only think I can see that a free list buys you is that if you have limited main memory and disks big enough that significant portions of the bit map couldn't be kept in the buffer cache, grabbing the first blcok off the free list rather than making an effort to pick an optimally positioned block would require fewer disk accesses (it would also require less CPU time under any circumstances). Since the days of 256K PDP-11s are behind us, I don't see the free list being worth much anymore... Guy Harris {seismo,ihnp4,allegra}!rlgvax!guy