linux-kernel - Re: Offset into block file for mounting?

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Message-ID: <20110103142210.GC6024@thunk.org>
Date:	Mon, 3 Jan 2011 09:22:10 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Tom Watson <tsw@...ana.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Offset into block file for mounting?

On Mon, Jan 03, 2011 at 05:13:04AM -0800, Tom Watson wrote:
> Hi...
> 
> I recently got a "floppy drive emulator" that uses an USB thumb drive to
> emulate floppy images to be used as a floppy drive.  In trying out the
> software (it is a bit cryptic), I found that this model stores the
> floppy images consecutively on said thumb drive (with some padding to
> make the drives on 1.5 Mbyte borders on the thumb drive.  When plugging
> in the thumb drive, the 'vfat' module just mounts the first image (as I
> expect).  What I would like to do is mount some of the other images that
> exist (I dumped out the raw thumb drive).  I believe that a simple way
> to do this is to add an offset to the vfat mount, so that the accesses
> begin at (as in my example) 0x180000 (and multiples thereof) from the
> start of the physical drive.
> 
> Is there any way to add such an offset?

Probably the easist way to solve this is to use devicemapper.  You can
use dmsetup[1] to create a device mapper volume that starts and ends at
the appropriate offsets.  You can then mount the device mapper volume
using anyu file system without having to hack in a offset mount
option.

[1] See the dmsetup man page for more details.

    			       	   - Ted
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