Tuning information for ISC 386/ix

John R. Levine johnl at esegue.segue.boston.ma.us
Wed Sep 27 15:32:59 AEST 1989

In article <38839 at bu-cs.BU.EDU> madd at 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

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 at esegue.segue.boston.ma.us, {ima|lotus}!esegue!johnl, Levine at YALE.edu
Massachusetts has 64 licensed drivers who are over 100 years old.  -The Globe

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