Path: utzoo!attcan!uunet!ginosko!gem.mps.ohio-state.edu!uwm.edu!rutgers!mit-eddie!genrad!decvax!ima!esegue!johnl
From: johnl@esegue.segue.boston.ma.us (John R. Levine)
Newsgroups: comp.unix.i386
Subject: Re: Tuning information for ISC 386/ix
Message-ID: <1989Sep27.053259.6557@esegue.segue.boston.ma.us>
Date: 27 Sep 89 05:32:59 GMT
References: <38451@bu-cs.BU.EDU> <14663@haddock.ima.isc.com> <14711@haddock.ima.isc.com> <38839@bu-cs.BU.EDU>
Reply-To: johnl@esegue.segue.boston.ma.us (John R. Levine)
Organization: Segue Software, Cambridge MA
Lines: 20

In article <38839@bu-cs.BU.EDU> madd@cs.bu.edu (Jim Frost) writes:
>I'm kind of curious about the 386/ix "FFS".  What I'd like to know is
>if the free list management has been improved ...

When FFS mounts a filesystem, it reads in the free list and keeps an
in-core bitmap of free space that it uses to allocate space.  When it
unmounts a filesystem it rewrites an updated free list based on the
bit map.  (In between it writes an empty free list so that in case of
a crash fsck will notice all the lost blocks and recreate the free list.)
It uses the bit map to do sensible file allocation.  It also does things
like noticing when it can do a large read for a contiguous bunch of
blocks.

A nice thing about this scheme is that the format of an unmounted disk
remains exactly the same as AT&T's SysV/386, so you can carry file systems
back and forth.
-- 
John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 492 3869
johnl@esegue.segue.boston.ma.us, {ima|lotus}!esegue!johnl, Levine@YALE.edu
Massachusetts has 64 licensed drivers who are over 100 years old.  -The Globe