I've been trying to better understand how the linux filesystem works, looking at journaling, inodes, and access control list. In looking into this, I came across filesystems which don't seem to act how I would expect a filesystem to act, such as glusterfs and mergerfs. Instead of getting written to a hard drive similar to how
mkfs.xfs would, they are run on top of the other filesystems. So both ext3 and mergerfs (or glusterfs) could be used with the same drive, which seems strange since I as far as I know, two filesystems can't be defined on the same partition.
Is my understanding of filesystems wrong, or is there something special about the mergerfs/glusterfs systems which distinguish them from ext3 or xfs?