To expand lxd’s default btrfs storage pool:

# lxc storage list
+---------++--------+--------------------------------+---------+
|  NAME   || DRIVER |             SOURCE             | USED BY |
+---------++--------+--------------------------------+---------+
| default || btrfs  | /var/lib/lxd/disks/default.img | 2       |
+---------++--------+--------------------------------+---------+

Once you know the location of the disk image, you must first expand the actual .img file.

# truncate -s +500G /var/lib/lxd/disks/default.img

Reboot the server to effect the changes to the loopback device (too lazy to figure out how to do this online – losetup -c didn’t work.)

Then, use btrfs to resize the filesystem within the .img

# btrfs filesystem resize max /var/lib/lxd/storage-pools/default

Reboot the server again to effect the changes to the loopback device.

You should see the new disk size on both host loopback device and container.

This doesn’t seem to update the configuration file within lxd, so the storage pool will still say the old size configuration value. I’m not sure if this has any effect though, since the btrfs storage mode seems to be pretty much passthrough.

btrfs resize documentation: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Storage_Administration_Guide/ch06s03.html

GitHub issue: https://github.com/lxc/lxd/issues/4847