Refer to : Resizing zfs filesystem in LXD container

Show pool status

 $ sudo zpool status -v
  pool: default
 state: ONLINE
  scan: scrub repaired 0B in 0 days 00:10:50 with 0 errors on Sun Aug 14 00:34:52 2022
config:

	NAME                                          STATE     READ WRITE CKSUM
	default                                       ONLINE       0     0     0
	  /var/snap/lxd/common/lxd/disks/default.img  ONLINE       0     0     0

errors: No known data errors

Stop instance and lxd service

$ lxc stop instance_name 
$ sudo snap stop lxd 

Increase volume

$ sudo truncate -s +20G /var/snap/lxd/common/lxd/disks/default.img

$ sudo snap start lxd 
$ sudo zpool set autoexpand=on default
$ sudo zpool online -e default /var/snap/lxd/common/lxd/disks/default.img
$ sudo zpool set autoexpand=off default