Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!ll-xn!mit-eddie!bloom-beacon!think!ephraim
From: ephraim@think.COM (ephraim vishniac)
Newsgroups: comp.sys.mac
Subject: Re: Physical Storage Amounts on Different Hard Disks (Size of Folders)
Message-ID: <20438@think.UUCP>
Date: 3 May 88 14:20:56 GMT
References: <1988May2.130641.357@mntgfx.mentor.com>
Sender: usenet@think.UUCP
Reply-To: ephraim@vidar.think.com.UUCP (ephraim vishniac)
Organization: Thinking Machines Corporation, Cambridge, MA
Lines: 46
Keywords: Hard disk folder storage size

In article <1988May2.130641.357@mntgfx.mentor.com> tomc@mntgfx.mentor.com (Tom Carstensen) writes:
>I came across a VERY interesting phenomena when I copied the
>data of my XP20 hard disk, to a CMS Pro 102k/II internal hard 
>disk on a Mac II.  (Note that the XP20 was create and used
>originally on a Mac SE).  The resulting two disks, both
>having IDENTICAL information on them showed a difference of
>over 1 Meg of storage:  XP20 18000K,  CMS 19200K.
>
>The CMS was formatted and initialized before the copy was done, 
>and had only  about 32K before the copy began.
>
>I noticed that each folder on the CMS we slightly larger
>than the same folder on the XP20.  This eventally added up 
>to 1M for all the folders.
>
>What is the reason for this.  Is it the interleave factor,
>(I think both are 1:1 - maybe not?  I know the CMS is) or
>something different about the CMS hard drive.
>
> . . I'm very curious whats adding 1Meg per 20Meg of space??

Interleaving has nothing to do with the capacity or usage of the disk.
In fact, interleaving has very little to do with anything except very
low-level performance tweaking.

Space on Macintosh volumes (as in many kinds of file systems) is not
necessarily allocated one sector at a time.  It's allocated in "Volume
Allocation Units."  The size of a VAU is proportional to the overall
size of the disk.  On HFS volumes up to about 32 megabytes (64K
sectors), the VAU is one sector.  So, it's the same on an 800K floppy
and on a 20M hard disk.  Above 32M, the VAU increases by roughly one
sector per 32M.

Your 102M disk probably has an allocation unit of 4 sectors (2K)
instead of 1 sector (.5K).  Statistically, expect a waste of .5 VAU
per file.  That's an increase of .75K per file going between the disks
you describe, so you'd see an increase of 1M with about 1300 files.
Sound about right?




Ephraim Vishniac					  ephraim@think.com
Thinking Machines Corporation / 245 First Street / Cambridge, MA 02142-1214

     On two occasions I have been asked, "Pray, Mr. Babbage, if you put
     into the machine wrong figures, will the right answers come out?"