Using qemu-nbd to mount a filesystems within a qcow2 file
Virt-manager stores images of its virtual machines at the location
/var/lib/libvirt/images by default.
Most users would prefer a separate partition or a directory within
one’s home directory.
For example I have a separate partition
$ df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sdb14 329584540 26947832 285821604 9% /qemuvm
So I make /var/lib/libvirt/images a link
$ pwd
/var/lib/libvirt
$ ls -l
lrwxrwxrwx 1 root root 7 Jun 6 17:47 images -> /qemuvm
Inside /var/lib/libvirt/images one normally finds .qcow2 files
$ ls images
MXahs.qcow2 debian.qcow2 lost+found
qcow2 is the default image file format used by virt-manager.
Inside those .qcow2 files are images of the guest VM’s
filesystem. It is possible to mount those filesystems buried within the .qcow2 file
using the qemu-nbd command.
The ndb stands for Network Block Device, a protocol used
for accessing disks or partitions across a network.
The program qemu-nbd comes in the package qemu-utils in Debian and Ubuntu.
Using qemu-nbd to mount a qcow2 disk image
There are 4 steps
1 . load the nbd kermnel module
# modprobe nbd max_part=8
2 . mount the qcow2 disk image
# qemu-nbd -c /dev/nbd0 --read-only /var/lib/libvirt/images/MXahs.qcow2
This makes a device /dev/nbd0
3 . List the partitions within the nbd device
# fdisk -l /dev/nbd0
Disk /dev/nbd0: 25 GiB, 26843545600 bytes, 52428800 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xe1b0ab5a
Device Boot Start End Sectors Size Id Type
/dev/nbd0p1 * 2048 49793023 49790976 23.7G 83 Linux
/dev/nbd0p2 49793024 52414463 2621440 1.3G 83 Linux
4 . Mount the required partition, in this case /dev/nbd0p1 is
the Linux partition
# mount /dev/nbd0p1 /mnt/mxahs
mount: /mnt/mxahs: WARNING: source write-protected, mounted read-only.`
It only allows a read-only mount because we specified read-only in step 2
We can now look at the MXahs filesysten
# ls /mnt/mxahs
bin dev home lib64 lost+found mnt proc root sbin tmp var
boot etc lib libx32 media opt pulse run sys usr
We see the MXahs root filesystem
5 . Cleanup
Unmount the filesystem
umount /mnt/mxahs
Disconnect thee nbd device
qemu-nbd --disconnect /dev/nbd0
Remove the module
rmmod nbd
Warning
One should only do this when the guest is not running, particularly if the mount is in rw mode.
Writing on a running guest filesystem can cause irreversible
damage to the qcow2 file.
Recommendation
A qemu-nbd mount is useful if you have done something
mistaken to the guests filesystem ( eg editing /etc/fstab)
and it will not boot.
One can mount the qcow2 file in rw mode and repair the guest filesystem so it will
boot. One needs to do this with due care, heeding the warning above.
It may also occasionally be useful to do a qemu-nbd mount in read only mode
in order to transfer files to or from the host, but there are other
ways of doing that.