There are multiple use cases for mergerfs, most commonly for mixing multiple (hetrogeneous) file systems into one single union. A good example is unifying your /tmp over tmpfs and physical block. ==================== tmpfs /ram-tmp tmpfs defaults 0 0 /ram-tmp:/media/SSHD/tmp-disk:/disk-tmp /tmp fuse.mergerfs x-systemd.requires=/media/SSHD,x-systemd.requires=/ram-tmp,defaults,use_ino,allow_other,default_permissions,nonempty,minfreespace=1G,category.create=all 0 0 ==================== * In the above example, we first create a tmpfs based file system * We also have 2 block based file systems: /media/SSHD/tmp-disk and /disk-tmp * Using mergerfs, we create a unified output, exported over /tmp This allows for a fast and efficient tmpfs based /tmp, while at the same time helps you from running out of file system space, if your requirement is beyond the limit of tmpfs size allocated for /tmp