Hi!
When you first setup LXD, you assign some space for all the containers. This space is called a storage pool. Here is mine. The storage pool is called (in my case) lxd, I am using the zfs storage driver and I have 51 containers in total.
$ lxc storage list
+------+-------------+--------+--------+---------+
| NAME | DESCRIPTION | DRIVER | SOURCE | USED BY |
+------+-------------+--------+--------+---------+
| lxd | | zfs | lxd | 51 |
+------+-------------+--------+--------+---------+
Let’s see some more details about this storage pool. We run lxc storage info with the name of the storage pool (in my case, lxd). It shows the total space that my containers are using (around 45GB), and also the total space that I have dedicated for the storage pool (around 215GB).
$ lxc storage info lxd
info:
description: ""
driver: zfs
name: lxd
space used: 45.20GB
total space: 215.88GB
used by:
containers:
...
In my case, I am using the ZFS storage driver, and I can use some ZFS commands to get a rough overall idea of how much space is used by each container. Below it reaffirms that I am using around 40-plus gigabytes for my containers, out of a total of about 200 gigabytes of available space.
$ zfs list
NAME USED AVAIL REFER MOUNTPOINT
lxd 42,3G 201G 24K none
lxd/containers 42,8G 201G 24K none
lxd/containers/c1 486M 201G 816M /var/snap/lxd/common/lxd/storage-pools/lxd/containers/c1
lxd/containers/c2 901M 201G 1,11G /var/snap/lxd/common/lxd/storage-pools/lxd/containers/c2
...
If you have setup LXD with the default settings, you probably created a storage pool of size 15GB (the default).
Can you verify that this is the case with your installation? Run the above commands and report back.
There are two ways to solve a small storage pool problem; you can either reinstall LXD and create a bigger storage pool, or you can create a second bigger storage pool, and move some containers there. Ask for details for any of these two.