Thanks, sounds interesting. So you create a snapshot, then let kopia process that snapshot rather than the live filesystem, and then remove the snapshot?

> I suppose if it only walked files it knew were changed it would be a lot faster.

Right, for me I'd want to set it up to do the full disk, so could be millions of files and hundreds of GB. But this trick should work with other backups software, so perhaps it's a viable option.

Exactly so.

Here's the script, should it be of benefit to someone, even if it of course needs to be modified:

    #!/bin/sh
    success=false
    teardown() {
      umount /mnt/backup/var/lib/docker || true
      umount /mnt/backup/root/.cache || true
      umount /mnt/backup/ || true
      for lv in root docker-data; do
        lvremove --yes /dev/hass-vg/$lv-snapshot || true
      done
    
      if [ "$1" != "no-exit" ]; then
        $success
        exit $?
      fi
    }
    
    set -x
    set -e
    teardown no-exit
    trap teardown EXIT
    for lv in root docker-data; do
      lvcreate --snapshot -L 1G -n $lv-snapshot /dev/hass-vg/$lv
    done
    
    mount /dev/hass-vg/root-snapshot /mnt/backup
    mount /dev/hass-vg/docker-data-snapshot /mnt/backup/var/lib/docker
    mount /root/.cache /mnt/backup/root/.cache -o bind
    
    chroot /mnt/backup kopia --config-file="/root/.config/kopia/repository.config" --log-dir="/root/.cache/kopia" snap create / /var/lib/docker
    kopia --config-file="/root/.config/kopia/repository.config" --log-dir="/root/.cache/kopia" snap create /boot /boot/efi
    success=true

Awesome, thanks!