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Mount a partition located inside a file or logical volume

Content

Tested on

Ubuntu (Xenial)

Objective

To mount a partition that is located inside a disc image file or an LVM logical volume.

Background

For physical storage devices, GNU/Linux-based operating systems will usually detect whether there is a partition table, and present any data partitions as separate block devices. For example, partition 1 of the block device /dev/sda would be presented as /dev/sda1.

This does not happen when the partition table is located inside:

This issue most commonly arises when the partitions have been created by a virtual machine which is using the file or logical volume as backing for a virtual hard drive. The VM will detect the partition table automatically, but should you wish to access the partition from outside the VM for any reason (for example, to copy data or reset passwords) then some means is needed to gain access to the partition inside the disc image.

Scenario

Suppose you wish to reset the root password of a virtual machine running GNU/Linux. It has one virtual disc, which is presented as /dev/vda inside the VM, and mapped to the logical volume /dev/vg0/foo outside the VM.

The virtual disc has a partition table containing three partitions. These are automatically detected by the operating system within the VM, and presented as /dev/vda1 (root), /dev/vda2 (home) and /dev/vda3 (swap), however the partitions are not visible from outside the VM.

Method

Overview

The method described here as three steps:

  1. Install the kpartx command (if not already present).
  2. Use kpartx to present the partitions as separate block devices.
  3. Mount the filesystem in the normal way.

Install the kpartx command

This command is provided by the kpartx package on both Debian-based systems:

apt-get install kpartx

and also on Red Hat-based systems, where it is usually installed by default.

Use kpartx to present the partitions as separate block devices

Use the -a (add) option to request that a new mapping be created for each partition found, and the -v (verbose) option to request a list of those mappings:

kpartx -av /dev/vg0/foo

If successful the output should be similar to the following:

add map vg0-foo1 (252:3): 0 1048576 linear 252:2 2048
add map vg0-foo2 (252:4): 0 524288 linear 252:2 1050624
add map vg0-foo3 (252:5): 0 522240 linear 252:2 1574912

The mappings are created in /dev/mapper/, so the first partition listed would be presented as /dev/mapper/vg0-foo1.

Mount the filesystem in the normal way

In this scenario it is the first partition to which access is wanted. Now that it has been presented as a separate block device, it should be possible to mount it in the normal way:

mkdir /mnt/vmroot
mount /dev/mapper/vg0-foo1 /mnt/vmroot

It is similarly possible to use commands such as mkfs, fsck, resize2fs, or anything else which operates on a partition.

Partitions presented using kpartx should not normally be listed in /etc/fstab, because the effect of kpartx does not persist across a reboot. If long-term access is required then the kpartx and mount commands would be need to be scripted to run at boot time.

Notes

There are two similarly named commands for mapping partitions: partx, which is part of the util-linux package, and a derivative named kpartx, which is part of multipath-tools. The main differences are that:

They accept similar arguments, and both have the ability to handle multiple types of partition table (including MBR and GPT). Note that whereas kpartx creates mappings in /dev/mapper/, partx places them in /dev/.

See also

Tags: lvm