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From: hedrick@topaz.rutgers.edu (Charles Hedrick)
Newsgroups: comp.unix.wizards
Subject: Re: Disk Fragmentation
Message-ID: <13521@topaz.rutgers.edu>
Date: Fri, 24-Jul-87 20:37:25 EDT
Article-I.D.: topaz.13521
Posted: Fri Jul 24 20:37:25 1987
Date-Received: Sat, 25-Jul-87 16:03:16 EDT
References: <8441@brl-adm.ARPA> <7674@mimsy.UUCP>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 27

In article <8441@brl-adm.ARPA> eichelbe@nadc.arpa (J. Eichelberger) writes:
>Is there a disk defragmenter utility for 4.3 BSD UNIX on VAXes?
>That is, a utility to move files around on the disk so that they are
>less fragmented (more contiguous) and larger free space areas exist on
>the disk.

In article <7674@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) replies:
>Yes.  It is called `cp'.

Chris is right, of course, but it is probably worth one more comment
about why he is right.  He alludes to the fancy allocation strategies
used by the Berkeley fast file system.  That's certainly important,
because it lets your system make the best use of free space.  You
don't need literally contiguous space in order to get reasonable
performance.  But their most important trick is the 90% rule.  Note
that fragmentation is a problem mostly when disks are very nearly
full.  If you just have a few blocks free, they'll probably be
scattered all over the place.  But if you have lots of free space,
with reasonable algorithms you can avoid fragmentation, and thus never
need to reorganize.  So what Berkeley did is to make sure that you
never fill your disk.  When you get to 90% of the capacity, the system
pretends it is full.  By never letting you completely fill the disk,
the fast file system makes sure that there is enough free space that
it can normally find a fairly good location for the next block of a
file.  Most of us think it is worth wasting 10% of our disk to avoid
having to do defragmentation all the time.  Most Unix installations
depend upon this, and don't do disk space reorganizations.