Today I needed to run a virtual machine on my laptop. It is not very powerful so I wanted to avoid as much overhead as possible: skipping the host's VFS/filesystem layer is fairly easy, you just have to give access to a raw partition the your virtualization software instead of a file. After all, both are just seen as a big array of bytes. My hard drive is under LVM, so I created a dedicated logical volume that I wanted to be writable by me given I planned to run VirtualBox as a user.
I knew I could do this with udev(8) but skimming through the documentation and fumbling its rules would have been too long for my available time, so I tried Google with no luck and finally asked on IRC, where I found that someone already did this for the same reason.
The configuration line is quite easy:
root@r2d2# cat /etc/udev/rules.d/99-my.rules
SUBSYSTEM=="block",KERNEL=="dm-*",ACTION=="add|change",ENV{DM_NAME}=="*-vbox*",GROUP="jlh"
This rules basically tells that for any add or change of a block device named "dm-*" and matching "*-vbox*", change its group to "jlh" (note that == and = are different, as in many programming languages). One interesting thing to note is that ENV{DM_NAME}=="*-vbox*" is an helper environment variable that is set by udev(8) standards rules. Those stand in /lib/udev/rules/ on Debian and udev(8) merges the content of this directory with the standard configuration directory /etc/udev/rules.d/. Rules are applied by filename order, so be careful to be the last one. Initially I used "90-my.rules" but there is a rule in "91-permissions.rules" that overrode mine. You can easily debug by running udevd --debug, although the output is quite verbose.
The next step is to create a VMDK file for VirtualBox that will point to the raw device and then attach is to your VM's storage controller. This is quite well documented in the manual (Using a raw host hard disk from a guest).
Basically:
jlh@r2d2$ VBoxManage internalcommands createrawvmdk \
-filename VirtualBox VMs/vm1/data.vmdk \
-rawdisk /dev/mapper/vg0-vbox_vm1
jlh@r2d2$ VBoxManage showvminfo vm1 | grep 'Storage Controller Name'
Storage Controller Name (0): IDE Controller
jlh@r2d2$ VBoxManage storageattach vm1 --storagectl "IDE Controller" --port 0 --device 0 --type hdd --medium /home/jlh/VirtualBox\ VMs/FreeBSD/data.vmdtk
Your mileage may vary if, for example, you have a different storage controller name, different port or device. The full "showvminfo" output will tell you which slot is available. Another solution is to add another storage controller, although VirtualBox will not permit you to have multiple IDE controller. You can add a S-ATA controller which allows you to plug up to 30 devices:
jlh@r2d2$ VBoxManage storagectl vm1 --name "SATA Controller" --add sata --controller IntelAHCI --bootable on jlh@r2d2$ VBoxManage storageattach vm1 --storagectl "SATA Controller" --port 0 --device 0 --type hdd --medium /home/jlh/VirtualBox\ VMs/boot.vmdk jlh@r2d2$ VBoxManage storageattach vm1 --storagectl "IDE Controller" --port 1 --device 0 --type hdd --medium /home/jlh/VirtualBox\ VMs/root.vmdk
VitualBox and VboxManage are pretty well documented.
