I have randomly been reading about union file system which enables a user to mount multiple filesystems on top of one another simultaneously.
However, am finding trouble deciding on which one to use(Unionfs vs Aufs vs Overlayfs vs mhddfs) and why as I have not found concrete information on the subject anywhere. I know for instance that overlayFS has been adopted in the mainstream Linux kernel which means it might get wider adoption. Would appreciate if someone would give me some perspective.
Also I can't find any conceiving use-case for Union file system over something like LVM (as recommended by users in separate question) or RAID setup except in the fact that LVM requires formatting all the drives which might not be desirable if you already have valuable data on the drives.