Update: it seems that kpartx pretty much does all of the below. Thanks to Faidon Liambotis for the pointer.
Every now and then, I have a disk image (as produced by
dd) and I need
to access separate partitions. Unfortunately, the patch allowing partitions
on loop devices to be accessed via their own device nodes does
not appear to be in the latest (Debian) 2.6.18 kernels — the
loop module does not have a
parameter, according to
So this time I sat down to come up with a recipe on how to
access the partitions, and after some arithmetic and much swearing
at disk manufacturers, and especially the designers of the
msdos partition table type, I think I have found the
solution, and the urge to document it for posterity.
It's all about the
-o parameter to
losetup, which specifies how many bytes into
the disk a given partition starts. Getting this number isn't
straight forward. Well, it is, if you know how, which is why I am
Let's take a look at a partition table, with sectors as units:
$ /sbin/fdisk -lu disk.img You must set cylinders. You can do this from the extra functions menu. Disk disk.img: 0 MB, 0 bytes 255 heads, 63 sectors/track, 0 cylinders, total 0 sectors Units = sectors of 1 * 512 = 512 bytes Device Boot Start End Blocks Id System disk.imgp1 * 63 96389 48163+ 83 Linux disk.imgp2 96390 2056319 979965 82 Linux swap / Solaris disk.imgp3 2056320 78140159 38041920 5 Extended disk.imgp5 2056383 3052349 497983+ 83 Linux disk.imgp6 3052413 10859939 3903763+ 83 Linux disk.imgp7 10860003 68372639 28756318+ 83 Linux disk.imgp8 68372703 76180229 3903763+ 83 Linux disk.imgp9 76180293 78140159 979933+ 83 Linux
The first few lines is
fdisk complaining not being
able to extract the number of cylinders, since it has to operate on
a file which does not provide an
The first important data are the units, which are stated to be 512 bytes per sector. We take note of this value as the factor for use in the next operation.
Let's say we want to access the 7th partition, which is 10860003
sectors into the disk, according to the
We know that each sector is 512 bytes, so:
10860003 * 512 = 5560321536
Passing this number to
losetup produces the desired
# losetup /dev/loop0 disk.img -o $((10860003 * 512)) # file -s /dev/loop0 /dev/loop0: Linux rev 1.0 ext3 filesystem data # mount /dev/loop0 /mnt [...] # umount /mnt # losetup -d /dev/loop0
If the partition really holds a normal filesystem, you can also
mount set up the loop device, and manage it
# mount -o loop,offset=$((10860003 * 512)) disk.img /mnt [...] # umount /mnt
And since there's aparently no means to automate the whole process for an entire disk, I hacked up plosetup. Enjoy:
# plosetup lapse.hda . I: partition 1 of lapse.hda will become ./lapse.hda_p1 (/dev/loop0)... I: plosetup: skipping partition 2 of type 82... I: plosetup: skipping partition 3 of type 5... I: partition 5 of lapse.hda will become ./lapse.hda_p5 (/dev/loop1)... I: partition 6 of lapse.hda will become ./lapse.hda_p6 (/dev/loop2)... I: partition 7 of lapse.hda will become ./lapse.hda_p7 (/dev/loop3)... I: partition 8 of lapse.hda will become ./lapse.hda_p8 (/dev/loop4)... I: partition 9 of lapse.hda will become ./lapse.hda_p9 (/dev/loop5)... # ls -l total 0 lrwxrwxrwx 1 root root 10 2006-10-20 13:25 lapse.hda_p1 -> /dev/loop0 lrwxrwxrwx 1 root root 10 2006-10-20 13:25 lapse.hda_p5 -> /dev/loop1 lrwxrwxrwx 1 root root 10 2006-10-20 13:25 lapse.hda_p6 -> /dev/loop2 lrwxrwxrwx 1 root root 10 2006-10-20 13:25 lapse.hda_p7 -> /dev/loop3 lrwxrwxrwx 1 root root 10 2006-10-20 13:25 lapse.hda_p8 -> /dev/loop4 lrwxrwxrwx 1 root root 10 2006-10-20 13:25 lapse.hda_p9 -> /dev/loop5 # plosetup -c . # ls -l total 0
(this post is dedicated to Penny for no other reason than the tunes I am listening to right now)
NP: Fly My Pretties / The Return of Fly My Pretties
Update: Be careful about the
$((...)) style arithmetic.
to overflow at 32bit.
to get it right. If in doubt, use perl or a calculator.