mkdir /tmp/lol
cd /tmp/lol
dd if=/dev/zero of=/tmp/lol/disk.img bs=1M count=2048
mkfs.ext4 /tmp/lol/disk.img

cp --sparse=never /tmp/lol/disk.img /tmp/lol/disk2.img
rm -f /tmp/lol/disk.img

cat > /tmp/lol/disk.conf <<DELIM
## Container
lxc.utsname                             = container1
lxc.rootfs                              = /tmp/lol/disk2.img
lxc.arch                                = x86_64
lxc.tty                                 = 6  # if you plan to use container with physical terminals (eg F1..F6)
lxc.pts                                 = 1024
lxc.cgroup.devices.allow                = a
lxc.mount.entry                         = proc proc proc nosuid,nodev,noexec  0 0
lxc.mount.entry                         = sysfs sys sysfs nosuid,nodev,noexec,ro 0 0
lxc.mount.entry                         = devpts dev/pts devpts nosuid,noexec,mode=0620,ptmxmode=000,newinstance 0 0
lxc.mount.entry                         = tmpfs dev/shm tmpfs nosuid,nodev,mode=1777 0 0
lxc.mount.entry                         = tmpfs run tmpfs nosuid,nodev,noexec,mode=0755,size=128m 0 0
lxc.mount.entry                         = tmpfs tmp tmpfs nosuid,nodev,noexec,mode=1777,size=1g 0 0
DELIM

lxc-start -n container1 -f /tmp/lol/disk.conf -C invalidcommand

df -h
rm -f /tmp/lol/disk2.img
df -h # disk space is the same
reboot
df -h # disk space returned to normal