I tried more or less the same procedure on a different LXD manager and something really weird happened. Initially, I had two containers on the default storage: taking only 18GB out of 30GB:
info:
description: ""
driver: btrfs
name: default
space used: 18.45GB
total space: 30.00GB
used by:
images:
- d42adae80c7bbe780b23d225d73dec2003b4f17fed0690706d4f1baf7e2c9d10
instances:
- container1
- container2
profiles:
- default
The total space of the disk is 160GB and outside LXD only 2GB were taken (total free space before the procedure: 160 - 18 -2 = 140GB).
I created a new storage pool bigstorage of 100GB. I then tried to move container2 on the new storage pool. The procedure took quite a while and I could see a progressbar showing:
Transferring instance: container2: 89.01GB (35MB/s)
Once the progressbar reached 89GB of transferred data, the same No space left on device error appeared and now I only have 40GB of free space left on the disk! I am wondering why LXD tried to transfer more than 90GB of data from the default storage (with size 30GB and only filled with 18GB of data, roughly half for container2 so I would expect at most 9GB of data transfer). Where did it find all this volume and where is it stored now?