In 3.2.3 I observed that I can unmount the underlying storage of a brick and GlusterFS happily chugs along and does not even log that its brick was unmounted.  Instead it keeps putting data into the same directory which normally means it would start putting data into the root filesystem of the server.  All the data that was in the mount is now gone and it risks filling up the root filesystem either with new data or from self-heal.

Although a brick's filesystem/disk/LV should not get unmounted for any sane reason I can think of, Gluster should prevent it from happening in the first place.  If a program relies on a resource, it should make sure that resource can't be taken away without throwing a fit about it and logging errors.

When a volume is started Gluster could see what bricks are in it and grab a filehandle on each which would prevent umount from unmounting it without a -f and tell the admin that the filesystem is in use (which it is).  When a volume is stopped it could simply release the handle.  The brick commands would also need that functionality.