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From: dca@toylnd.UUCP (David C. Albrecht)
Newsgroups: comp.sys.att
Subject: Re: Missing: 5 MB on a Unix PC.
Message-ID: <183@toylnd.UUCP>
Date: Tue, 3-Nov-87 03:20:12 EST
Article-I.D.: toylnd.183
Posted: Tue Nov  3 03:20:12 1987
Date-Received: Tue, 10-Nov-87 01:44:04 EST
References: <381@sdcjove.CAM.UNISYS.COM>
Distribution: comp.sys.att
Organization: Dave & Anne in Charlottesville, VA
Lines: 25
Summary: More info.

In article <381@sdcjove.CAM.UNISYS.COM>, john@CAM.UNISYS.COM (John Dempsey) writes:
In re-scanning your question I noticed I didn't really answer it very well.
First off I'm not really sure why the accuracy of the free space computation
seems to be off I don't know from whence they are getting their answer.
Some part of your 20 megabytes is missing because it is stored in an
invisible partition which is used as swap area.  This amount, however,
is never seen as part of the file system so would never show in a df or du.

Also you totally ignored in your computations the existence of inodes
(which are reported by df).  Files in the system are not actually serial
on disk but rather blocks indirectly referenced by 'inodes'.  I would have
to refer back to Unix literature to give you a precisely correct answer
but an inode contains some relatively small number of slots to directly
access the first 'x' blocks of a file.  Above that number it indirectly
gets the next block of the file through another inode block.  I believe one
slot is reserved for triple indirection.  Every file in the system
therefore requires at least one inode and maybe more.  Certainly these
structures take not inconsiderable space just a rough guess (from inode(4))
places them at about 64 bytes per.

As I'm using a 67M 3b1 I hopped over to my girlfriend's 20M machine to get
some numbers for you.  Her machine has a df of 6184 and a du at '/' of 21280.
The system lists 21% or 3092K free.

David Albrthere