Resizing an Encrypted Btrfs Disk Image

04/04/2019

Step 1. Unmount and Close

...if needed. Also backup your original image file:

$ sudo umount /dev/mapper/LUKSVOL
$ sudo cryptsetup close LUKSVOL
$ cp LUKSVOLUME.img LUKSVOLUME.img.bak

Step 2. Expand the Container

Note that to calculate the additional space you want to add, multiply the 'byte size' (bs) times the count, e.g.: 1M * 100 = 100M.

$ dd if=/dev/zero of=LUKSVOLUME.img bs=1M count=100 oflag=append conv=notrunc

Step 3. Open and Resize the Encrypted Volume

$ sudo cryptsetup open LUKSVOLUME.img LUKSVOL
$ sudo cryptsetup resize -v LUKSVOL

Step 4. Expand the Partitionless Btrfs FS

I.e. with a 'whole disk' fs.

a. Mount the Btrfs FS

$ sudo mount /dev/mapper/LUKSVOL /mnt

b. Create a Temporary Loop Device

Be sure it's as large or larger than the old btrfs device (i.e. before it was appended in step 1):

$ sudo dd if=/dev/zero of=/tmp/btrfs-vol1.img bs=1M count=50
$ sudo losetup -f /tmp/btrfs-vol1.img
$ losetup # check the new device id

c. Add the New Loop Device to the Btrfs FS

$ sudo btrfs device add /dev/loopX /mnt/
$ sudo btrfs fi show /mnt/ # verify that the new device is added

d. Remove and Re-Add Devices

Removing and re-adding the original device allows the parent btrfs fs to update the storage capacity (I wasn't able to achieve this with scrub or balance or any other method I tried). After, the loop device can be removed from the btrfs fs:

$ sudo btrfs device delete /dev/mapper/LUKSVOL /mnt/
$ sudo btrfs device add /dev/mapper/LUKSVOL /mnt/
$ sudo btrfs device delete /dev/loopX /mnt/
$ sudo btrfs fi show /mnt/ # verify proper device(s)

d. Balance and Scrub

To be safe:

$ sudo btrfs fi balance start /mnt/
$ sudo btrfs scrub start /mnt/

Then you can unmount and cryptsetup close, and your image file should be properly resized.