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From: dprrhb@inetg1.ARCO.COM (Reginald H. Beardsley)
Newsgroups: comp.os.coherent
Subject: Re: hard drive prob
Message-ID: <1991May22.135918.8620@Arco.COM>
Date: 22 May 91 13:59:18 GMT
References: <1991May17.192248.26140@spool.cs.wisc.edu> <858@cadlab.sublink.ORG> <1991May21.222023.27809@milton.u.washington.edu>
Sender: dprrhb@inetg1 (Reginald H. Beardsley)
Organization: ARCO Oil & Gas Company
Lines: 40

In article <1991May21.222023.27809@milton.u.washington.edu>, erich@milton.u.washington.edu (Eric Horst) writes:

> 
> That is one of the wonderful things about unix!  Multiple physical disks
> which may contain multiple logical partitions can be mounted together into
> one huge filesystem. 

    This is true only if you are talking about the LOGICAL file system.
Each partition has its own PHYSICAL file system with  its own private set
of inodes.  This is the reason for the proscription on hard links across 
file systems (ie. partitions and devices ), why you can't use "mv" across 
file systems, etc.

> I think that you are thinking of the more complex matter of making
> multiple physical disks appear as one large physical disk. (As in
> Netware 3.x, for example).  It CANNOT do this.  You cannot have files
> that exceed the size of the single physical disk or partition.

    Making multiple partitions appear as a single physical disk demands 
that ALL inodes in the system be uniq.  It can be done, but has its own
set of problems.

> One more time, the partition on the 2nd disk CANNOT be another file-system
> it must be mounted onto the /root filesystem forming one large, physically 
> transparent filesystem.

    One more time,  each partition has its own PHYSICAL file system.  Mounting
these file sytems on each other produces a large LOGICAL file system.  The mounts can be anywhere you want them.  They just have to be somewhere.  And it
is very definitely NOT transparent, unless you modify things like "mv" to do a copy and delete operation whenever it encounters an operation across file systems. 

For example, if you create 4 partitions that are the same size and run mkfs on 
each partition with same parameters (except of course, the partition number) you will have 4 filesystems, each of which will have EXACTLY the same list of inodes.  Your logical file system will then have 4 entries for each inode number.  Since links reference the inode directly the proscription on hard links across file systems arises.  For this reason symbolic links were created.


-- 
Reginald H. Beardsley       
ARCO Information Services
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