How to unmount and possibly delete old Ubuntu Core snap

When I type:

$ mount|grep snap
/var/lib/snapd/snaps/ubuntu-core_352.snap on /snap/ubuntu-core/352 type squashfs (ro,relatime)
/var/lib/snapd/snaps/ubuntu-core_423.snap on /snap/ubuntu-core/423 type squashfs (ro,relatime)
/var/lib/snapd/snaps/ubuntu-core_122.snap on /snap/ubuntu-core/122 type squashfs (ro,relatime)

I see several old snap "mounted" and available while only the latest version is used as current in /snap/ubuntu-core

It's not a big deal but do I need older ubuntu core ? Isn't this a bug there are not deleted ? Could I simply unmount and delete the files ?

  sudo snap remove core --revision 122
  sudo snap remove core --revision 352

So, basically passing the revisions to remove in the command line.

To check the ones you've installed you have to use snap list [snap-name] --all

To completley remove snap core, snap and mounted filesystems:

1. List installed snap cores snap list --all should give you something like:

   Name Version Rev Tracking Publisher Notes core 16-2.41 7713 stable canonical✓ core,disabled core 16-2.42 7917 stable canonical✓ core

2. Remove disabled ones first snap remove core --revision 7713 notice the Rev column in step 1 should match --revision

3. Uninstall snapd (on Ubuntu: apt-get purge snapd)

4. Findout if snap is still mounted (for example df -H) gives something like
   /dev/loop0 94M 94M 0 100% /snap/core/7917 and unmount it umount /snap/core/7917 but it shouldn't

5. If there are still mounted fs-es, you should attempt manual cleanup of /etc/systemd/system/snap-core-* something files.

Next time when your server reboots, snaps won't be there, unless you install/remove snapd again from some reason...

So I unmount the old snap:

sudo umount /snap/ubuntu-core/122

and then done some manual clean up

sudo rmdir /snap/ubuntu-core/122
sudo rm /var/lib/snapd/snaps/ubuntu-core_122.snap

The same for the other one. The sky did not fall... yet