How to remove multiple links to the same directory?

Question

I have a broken lxd installation and want to completely delete it and start over, but there's a problem. /var/lib/lxd/devices/desktop contains a bunch of subdirs that are linked to directories elsewhere in the filesystem (even on different drives) such as

drwxr-xr-x 28 karl   karl       4096 Apr 11 15:40 disk.shareName.home-karl
drwxr-xr-x  4 nobody nogroup    4096 Apr 14 12:07 disk.thor.mnt-thor
drwxr-xr-x  2 root   root       4096 Mar 29 14:02 disk.usbdrive.mnt-usbdrive

For example, if I delete a file from disk.thor.mnt-thor, it also gets deleted from /home/storage (both directories point to the same place). disk.usbdrive.mnt-usbdrive shows all contents of my usb drive mounted to /mnt/usbdrive. Both directories show its contents, and modifications in one are reflected in the other.

But now if I want to delete these directories, I have a problem, because I just want to remove those directories under lxd, not the actual directories they point to and not their contents. I tried using unlink, but it doesn't work on directories.

Answer 1

Try umount-ing them.

The VFS layer in Linux allows files and directories to be "bind-mounted" to another location. The result indeed looks like a hardlink, but doesn't actually exist on disk – bind mounts are in-memory just like regular mounts are.

This feature also means that a single filesystem can be mounted on several places at once. For example, the same /dev/sda1 could be mounted at /mnt/usbdrive and on /var/lib/lxd/mnt-usbdrive.

All such magic mounts can be seen by running findmnt or mount.

(Windows and FreeBSD have similar features as well, e.g. nullfs.)