perbu (perbu) wrote,

tool tip: kpartx

kpartx is not, despite its name, some KDE-tool for partitioning disks. It is usually shipped with multipath - but it makes a lot of sense on its own. Its purpose is to read the partition table of a device or a file and create device map entries for the individual partitions.

So, how does it work? Say someone shippet you a DVD image with a xen instance on. You would like to extract a small file from the image but you don't want to go through the pain of installing xen and booting.

Lets see how it works. The image is contained in test7disk1.
root@nice:/media/cdrom# sfdisk -l test7disk1 
Disk test7disk1: cannot get geometry

Disk test7disk1:
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 = cylinders of 8225280 bytes, blocks of 1024 bytes.

   Device Boot Start     End   #cyls    #blocks   Id
test7disk1p1   *      0+     12      13-    104391   83
test7disk1p2         13     508     496    3984120   8e
test7disk1p3          0       -       0          0    0
test7disk1p4          0       -       0          0    0

Okey. Theres a bit of whining because sfdisk can't figure out the geometry. Lets ignore that.

Now lets add the maps:
root@nice:/media/cdrom# kpartx -a test7disk1 

Thats it! kpartx has set up a loopback device and created /dev/mapper/loop0p1 and /dev/mapper/loop0p2. These can be mounted, used av PVs in LVM or dealt with in other ways. Nice, eh?

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