CentOS - Aug 2006 - question on mounting a partition that is in a disk image

How do I mount a partition that is in an image file?

I have a file called centos.img that has 3 partitions in the file.
I need to copy data to the third partition on that image file.

I have seen things about a loop back device (which is fine)
but then it talked about an offset parameter and I dont know what
that is or more importantly what number to use. I hope I'm on the
right track.

How do I mount it?

THanks,

Jerry

On Wed, 2006-08-23 at 10:02 -0400, Jerry Geis wrote: 
> How do I mount a partition that is in an image file?
> 
> I have a file called centos.img that has 3 partitions in the file.
> I need to copy data to the third partition on that image file.
> 
> I have seen things about a loop back device (which is fine)
> but then it talked about an offset parameter and I dont know what
> that is or more importantly what number to use. I hope I'm on the
> right track.
> 
> How do I mount it?
Just assign the whole thing to a loop device. Then do an 

    "sfdisk -l /dev/loop0" # or whichever loop you used

If it is truly an image file, at that point it *appears* identical to a
real device. I've added little sample I ran, since I know this can be
scary.

Offset is just if you need to start elsewhere from the beginning. If
you're using LVM, you'll have more work to do.

===================================================================# Pardon my
verbosity as I set up a test scenario.
# lvextend --extents +40 VolGroupTemp/LogVol00
  Extending logical volume LogVol00 to 1.62 GB
  Logical volume LogVol00 successfully resized

mke2fs -b 4096 -m 0 -N 128 /dev/VolGroupTemp/LogVol00
<snip extremely fascinating refs to momentous cosmological events>

Maximum filesystem blocks=436207616
...
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912

Writing inode tables: done
Writing superblocks and filesystem accounting information: done
...

# mount -t ext2 /dev/VolGroupTemp/LogVol00 /VolGroupTemp/LogVol00/
# dd if=/dev/hda of=/VolGroupTemp/LogVol00/hda_image bs=8225280
<snip>
# losetup /dev/loop0 /VolGroupTemp/LogVol00/hda_image

[root at server01 ~]# sfdisk -l -uS /dev/loop0
Disk /dev/loop0: cannot get geometry

Disk /dev/loop0: 0 cylinders, 0 heads, 0 sectors/track
Warning: The partition table looks like it was made
  for C/H/S=*/255/63 (instead of 0/0/0).
For this listing I'll assume that geometry.
Units = sectors of 512 bytes, counting from 0

   Device Boot    Start       End   #sectors  Id  System
/dev/loop0p1   *        63    208844     208782  83  Linux
/dev/loop0p2        208845   2506139    2297295  8e  Linux LVM
/dev/loop0p3             0         -          0   0  Empty
/dev/loop0p4             0         -          0   0  Empty

# dc
63
512*pq
32256

# losetup -o 32256 /dev/loop1 hda_image
# e2fsck /dev/loop1
e2fsck 1.35 (28-Feb-2004)
/boot: clean, 43/26104 files, 19763/104388 blocks

# mkdir /mnt/temp
# du -s /mnt/temp/*  # Snipped below for brevity
51      /mnt/temp/config-2.6.9-34.0.1.EL
211     /mnt/temp/grub
986     /mnt/temp/initrd-2.6.9-34.0.1.EL.img
...
1442    /mnt/temp/vmlinuz-2.6.9-34.0.2.EL
1442    /mnt/temp/vmlinuz-2.6.9-34.EL

# dc
32256
208782
512*+pq
106928640
# losetup -o 106928640 /dev/loop2 hda_image
# pvdisplay /dev/loop2
  Found duplicate PV 602X4Js851U78eRzvYP3Fhyf4F8A1v68: using /dev/hda2
not /dev/loop2
  Found duplicate PV 602X4Js851U78eRzvYP3Fhyf4F8A1v68: using /dev/loop2
not /dev/hda2
  --- Physical volume ---
  PV Name               /dev/loop2
  VG Name               VolGroup00
  PV Size               1.09 GB / not usable 0
  Allocatable           yes (but full)
  PE Size (KByte)       32768
  Total PE              35
  Free PE               0
  Allocated PE          35
  PV UUID               602X4J-s851-U78e-RzvY-P3Fh-yf4F-8A1v68
================================================================
> 
> THanks,
> 
> Jerry
> <snip sig stuff>
HTH
-- 
Bill
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> How do I mount a partition that is in an image file?
>
> I have a file called centos.img that has 3 partitions in the file.
> I need to copy data to the third partition on that image file.
>
> I have seen things about a loop back device (which is fine)
> but then it talked about an offset parameter and I dont know what
> that is or more importantly what number to use. I hope I'm on the
> right track.
>
> How do I mount it?
Partitions in an image file.  Interesting.  I would first mount the image 
.. "mount centos.img /some/dir -o loop"

If that doesn't work .. I would need to know more about the partitioning 
and the image .. how was it created ... are there filesystems on these 
partitions, and if so, what are they?

Hope this helps,
Barry