unionfs(4)

unionfs4

Union mount file system

Usage

option UNIONFS

In loader.conf(5): unionfs_load="YES"

Overview

The UNIONFS driver is an implementation of a stackable unification filesystem.

Releases

3 releases since FreeBSD 15.0-RELEASE; the text changed 1 time.

  1. 15.0–15.1
  2. 16.0

Standards

J. S. Pendry and M. K. McKusick, Union mounts in 4.4BSD-Lite, Proceedings of the USENIX Technical Conference on UNIX and Advanced Computing Systems, December 1995.

P. H. Kamp and R. N. M. Watson, Jails: Confining the omnipotent root, Proceedings of the Second International System Administration and Networking Conference (SANE2000), May 2000.

History

The unionfs device driver first appeared in FreeBSD 5.0.

Authors

The unionfs device driver was written by Jan-Simon Pendry for 4.4BSD and Masanori OZAWA ⟨[email protected]⟩ reimplemented the handling of the locking for FreeBSD 7.0. The manual page was written by Gordon Bergling ⟨[email protected]⟩.

Bugs

Please see the mount_unionfs(8) manual page for a list of bugs regarding the unionfs filesystem.

From base 16.0-CURRENT-20261027-61c70dc16a79 in FreeBSD 16.0-CURRENT (2026-10-27). Parsed by carta from the page's mdoc source.