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