Union mount

In computer operating systems, union mounting is a way of combining multiple directories into one that appears to contain their combined contents.^[1] Union mounting is supported in Linux, BSD and several of its successors, and Plan 9, with similar but subtly different behavior.

As an example application of union mounting, consider the need to update the information contained on a CD-ROM or DVD. While a CD-ROM is not writable, one can overlay the CD's mount point with a writable directory in a union mount. Then, updating files in the union directory will cause them to end up in the writable directory, giving the illusion that the CD-ROM's contents have been updated.^[1]^[2]

^ ^a ^b Cite error: The named reference usenix was invoked but never defined (see the help page).
^ Cite error: The named reference lwn1 was invoked but never defined (see the help page).

[usenix-1] Cite error: The named reference usenix was invoked but never defined (see the help page).

[lwn1-2] Cite error: The named reference lwn1 was invoked but never defined (see the help page).

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