[sclug] Mount whole-disk image?

Dickon Hood sclug at splurge.fluff.org
Sun Jan 27 00:42:02 UTC 2008

On Sat, Jan 26, 2008 at 22:18:20 +0000, John Stumbles wrote:
: I know if I make an image of a filesystem with
: 	dd if=/dev/sda1
: (say) I can then mount it loopback and access it as a filesystem. Can I 
: do something similar with an image of an entire disk, e.g. made with
: 	dd if=/dev/sda
: ?

Quick answer: yes, but.

Slightly longer answer: almost certainly, but it will require work.

Full monty: if you have put the filesystem on the disc without a partition
table, then this will work happily.  If you have a standard PC partition
table, or similar, on the front, then it won't.

Your problem is that mount wants a set of blocks to look at which define
a filesystem.  Normally, the kernel parses the partition table for you,
and creates a block device (and character devices on most other Unices)
corresponding to each partition.  However, with a loopback mount that
doesn't happen.

What you *can* do is parse the partition table yourself, and pass '-o
offset' to losetup manually.  That will allow you to setup a loopback
device that's an offset into the file you pass it.  When you 'mount
-oloop' (or let mount determine that itself) it does a lot of magic with
losetup that's hidden from you.

I might be wrong, as it's been a long time since I needed to do anything
like this, but I think that's your best bet.  fdisk on the file may
produce something useful.  A quick 'dd if=/dev/sda of=test; ^C; fdisk
./test;' gave me enough to locate the filesystem within the image, albeit
with some whingeing about cylinders.

Dickon Hood

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