So I found myself in the position of wanting to poke around the file system of a virtual Windows machine – the kind of poking around you would prefer I didn’t if it were your Windows machine – and needed to make a VDI disk image available as a block device under Linux so it could be mounted in the normal fashion.
Googling around found some instructions; which didn’t work properly. Solutions were also available, but I’m writing up the ‘fixed’ instructions here to save myself time in case I need it again.
First step is to become root; if you need help doing that, this is probably the wrong page for you!
Next step is to install the Debian package qemu-utils which contains the tool qemu-nbd which we’ll need later. We also need to load the network block device module with a parameter :-
apt-get install qemu-utils modprobe nbd max_part=16
This parameter is the key here – for some reason the default on at least some of my Linux machines is not to create additional block devices for any additional partitions that show up.
The next step is to ‘attach’ the VDI image (or presumably anything supported by qemu-img which covers pretty much everything popular) and tell the Linux kernel that there may be some new partitions to create device files for :-
qemu-nbd -n -c /dev/nbd0 disk.vdi partx -a /dev/nbd0 partx: /dev/nbd0: error adding partitions 1-2
(Added the “-n” flag after reading about some more problems and a work-around; as I haven’t tested it, be careful!)
The error from partx indicates that qemu-nbd managed to create the partitions itself, but there are hints that this sometimes doesn’t happen so I’ve included the command here “just in case”. Once the partition block devices are present, they can be used as any ordinary devices.
Once finished, unmount anything mounted and release the block device with :-
qemu-nbd -d /dev/nbd0 rmmod nbd