Union file systems, such as unionfs and aufs, were inspired by Plan9 union directory mounts.
For example, they are used on live CDs to merge
/usr/bin from the CD with a writable file system, so that you can make changes to
/usr/bin, even tho the CD is read-only.
Union file systems: Implementations, Part I on lwn.net
For example, if I understand the docs correctly, on Plan9, you could do:
bind -b /usr/bin /bin
bind -b /usr/local/bin /bin
bind -b /home/username/bin /bin
And all the files in all three directories would appear in
/bin (in case of duplicate names, the one in the last-specified directory wins, due to the
I'm not sure if this is what Bruce means by "mount/filesystem namespace", or is something different.
You could probably also say that sshfs was inspired by Plan9's ftpfs.