Re: [Qemu-devel] block device loopback

[NyOS <lista@nyos.homelinux.net>]

On Thu, 01 Jun 2006 18:08:56 +0200, s[e]th & h[o]lth  
<seth.holth@gmail.com> wrote:

> Hello,
>
> I'm a qemu user and I'm looking for a way to use my virtual target drive
> inside my host computer.
> I've created a file that I'll call hda.disk like this : "dd if=/dev/zer  
> of=
> hda.disk count=1 bs=1M seek=1023" and i use it like "qemu -hda hda.disk"
> The problem i meet is that i can't use this file outside the virtual  
> machine
> and I'm trying to program a block device driver to read this file as a  
> IDE
> drive.
> The aim of this driver is quite similar to the loop driver but instead of
> "emulating" a partition, it emulates a IDE Hard Drive and give an access  
> to
> it via /dev/vdX and it's partition via /dev/vdXpY.
> Perhaps you will wonder why i use the qemu's mailing-list for this  
> problem :
> - i don't know exactly where i can find a way to achieve this project ;
> - i wonder how you read your hda.disk without using qemu ;
> - i would like to know how qemu fix the virtual drive property inside the
> virtual machine (cylinders, sectors, etc...) ;
> - i don't know anyone interested by this problem and quite good enough  
> linux
> driver developer to help me.
>
> Thank you very much and have a nice day !

Hi!

how many partitions do you have on that virtual disc image? maybe 1 is  
enough in most cases..
once i've read on a forum the next 'trick':

nyos@nyos:~$ /sbin/fdisk $HOME/image/qemu-win98.img
You must set cylinders.
You can do this from the extra functions menu.

Command (m for help): p

Disk /home/nyos/image/qemu-win98.img: 0 MB, 0 bytes
128 heads, 63 sectors/track, 0 cylinders
Units = cylinders of 8064 * 512 = 4128768 bytes

                           Device Boot      Start         End      Blocks    
Id  System
/home/nyos/image/qemu-win98.img1   *           1         520      
2096608+   c  W95 FAT32 (LBA)
Partition 1 has different physical/logical endings:
      phys=(518, 127, 63) logical=(519, 127, 63)

Command (m for help): q

so, there are 63 sectors/track, one track is 512 bytes long, and the 1st  
partition starts at 1th track
63*512*1=32256

so the following command:
sudo mount $HOME/image/qemu-win98.img $HOME/win98loop -o loop,offset=32256  
-t vfat
will mount the 1st partition in that image into the apropriate directory  
by ignoring the first 32256 bytes of the image leaving only the partition  
in it..

hope it helps

------------------------------------------------------------
Light thinks it travels faster than anything but it is wrong. No matter  
how fast light travels it finds the darkness has always got there first,  
and is waiting for it. - Terry Pratchett