I wanted to retrieve a file from a VirtualBox VDI image for this blog post.
I followed these instructions and ended up here:
Once having used nbd0, only rebooting the system makes it possible to mount another image ... a little bit unpractical.
What happened was this:
# modprobe nbd # NOO! Don't *EVER* do that!
# qemu-nbd -c /dev/nbd0 file.vdi
# kpartx -d /dev/nbd0
# mount /dev/nbd0… EHI! Where's /dev/nbdpp1 ??
# qemu-nbd -d /dev/nbd0
# rmmod nbd
rmmod: ERROR: Module nbd is in use
# kpartx -d /dev/nbd0
read error, sector 0
llseek error
llseek error
llseek error
# rmmod nbd
rmmod: ERROR: Module nbd is in use
# WHAT THE…
It turns out it's really modprobe nbd max_part=16, otherwise max_part
defaults to, uhm, zero? really? and kpartx cannot create device mappings
because there are not enough (as in, not even a single one) partition devices
available.
At this point, however, kpartx did create some mappings connected to, uhm, probably Ancient Beings from beyond spacetime, and because of those the device is in use and cannot be removed, and unmapping doesn't work either because the Ancient Beings from beyond spacetime are keeping the device busy by feeding on it.
I energized the pentacle and tried a desperate ritual of banishment:
# # Reconnect nbd0 to the vdi file to Restore the Balance
# qemu-nbd --verbose -c /dev/nbd0 file.vdi
# # This works now
# kpartx -vd /dev/nbd0
del devmap : nbd0p5
del devmap : nbd0p2
del devmap : nbd0p1
# # This too, the Ancient Beings lie asleep yet again
# modprobe nbd -r
At this point I managed to get my file, almost:
# modprobe nbd max_part=16
# qemu-nbd --verbose -c /dev/nbd0 file.vdi
NBD device /dev/nbd0 is now connected to file.vdi
# kpartx -va /dev/nbd0
add map nbd0p1 (254:12): 0 60260352 linear 43:0 2048
add map nbd0p2 (254:13): 0 2 linear 43:0 60264446
add map nbd0p5 (254:14): 0 2648064 linear 43:0 60264448
# mount /dev/nbd0p1 /mnt
mount: /dev/nbd0p1 is already mounted or /mnt busy
# # WHAT NOW?!
# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
…
nbd0 43:0 0 30G 0 disk
├─nbd0p1 43:1 0 28.8G 0 part
├─nbd0p2 43:2 0 1K 0 part
├─nbd0p5 43:5 0 1.3G 0 part
├─nbd0p1 254:12 0 28.8G 0 part
├─nbd0p2 254:13 0 1K 0 part
└─nbd0p5 254:14 0 1.3G 0 part
# # WHAAAT?!!
# kpartx -vd /dev/nbd0
del devmap : nbd0p5
del devmap : nbd0p2
del devmap : nbd0p1
# lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
…
nbd0 43:0 0 30G 0 disk
├─nbd0p1 43:1 0 28.8G 0 part
├─nbd0p2 43:2 0 1K 0 part
└─nbd0p5 43:5 0 1.3G 0 part
# mount /dev/nbd0p1 /mnt
# # I got my file, my preciouss file!
# umount /mnt
# kpartx -vd /dev/nbd0
# qemu-nbd -d /dev/nbd0
# rmmod nbd
# # sit in a corner hugging my precious file and sobbing quietly
As can be seen from the multiple exclamation marks, those Ancient Beings from beyond spacetime did manage to have a bite on my sanity after all.