increase a KVM guest's disk space
October 17, 2018 - Reading time: 2 minutes
  1. stop the VM
  2. run qemu-img resize vmdisk.img +10G to increase image size by 10Gb
  3. start the VM, resize the partitions and LVM structure within it normally

Online Method (using qemu, libvirt, and virtio-block)

For better or worse, the commands below will run even if the target virtual disk is mounted. This can be useful in environments where the disk cannot be unmounted (such as a root partition), the VM must stay on, and the system owner is willing to assume the risk of data corruption. To remove that risk, you would need to log into the VM and unmount the target disk first, something that isn't always possible.

Perform the following from the KVM hypervisor.

  1. Increase the size of the disk image file itself (specify the amount to increase):

    qemu-img resize <my_vm>.img +10G

  2. Get the name of the virtio device, via the libvirt shell (drive-virtio-disk0 in this example):

    virsh qemu-monitor-command <my_vm> info block --hmp
  drive-virtio-disk0: removable=0 io-status=ok file=/var/lib/libvirt/images/<my_vm>.img ro=0 drv=raw encrypted=0
  drive-ide0-1-0: removable=1 locked=0 tray-open=0 io-status=ok [not inserted]

  3. Signal the virtio driver to detect the new size (specify the total new capacity):

    virsh qemu-monitor-command <my_vm> block_resize drive-virtio-disk0 20G --hmp

Then log into the VM. Running dmesg should report that the virtio disk detected a capacity change. At this point, go ahead and resize your partitions and LVM structure as needed.

mount encrypted volumes from command line
September 19, 2015 - Reading time: 4 minutes

Unlocking and mounting the disk with udiskctl

Instead, I used udisksctl, a command-line interface that interacts with the udisksd service.

Here's what worked (/dev/sdb5 is the partition on my hard disk marked as crypt-luks):

udisksctl unlock -b /dev/sdb5
udisksctl mount -b /dev/mapper/ubuntu--vg-root

After typing the first command, you'll be prompted for your encryption passphrase. Once the encrypted partition is unlocked, the second command will mount it. If that's successful, you'll end up with a message similar to this:

Mounted /dev/dm-1 at /media/dpm/e8cf82c0-f0a3-41b3-ab28-1f9d23fcfa72

From there I could access the data :)

Locking the disk with udiskctl

Unmount the device:

udisksctl unmount -b /dev/mapper/ubuntu--vg-root

You'll need to deactivate all logical volumes in the ubuntu-vg volume group first. Otherwise you'll get an error along the lines of 'Device busy' if you try to lock it (more info):

sudo lvchange -an ubuntu-vg

Then you'll be able to lock back the encrypted partition

udisksctl lock -b /dev/sdb5

Notes

  • The udisksctl commands are executed without sudo.

  • Device mapper names: the ubuntu--vg-root naming might change across Ubuntu releases (e.g. I've seen it called system-root and ubuntu-root too). An easy way to find out the name is to run the following command after unlocking the LUKS partition:

    ls -la /dev/mapper

    Then looking at the output of the ls command, the name you'll need will be generally the one symlinked to /dev/dm-1

  • Device mapper names, alternative: an alternative to the previous command is to run:

    lsblk -e7

    There you'll be able to see the device name mapping as a tree view. The -e 7 option is used to exclude the loop devices (ID 7) created by installed snaps from the output. Simply to have less clutter.

  • Logical volume names: you can run the sudo lvs command to find out the names of volume groups and logical volumes
  • Disks app: the GNOME Disks app does not automatically deactivate the logical volumes before locking the partition. Even if you've successfully unlocked the partition via the GUI, you will need to go to the command line and execute the sudo lvchange -an ubuntu-vg command before you can lock it from the GUI.

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